[Objective] Algal polysaccharide is used as a curing agent in place of traditional cement, with the addition of corn fiber, water-retaining agents, and ecological fertilizers, to improve the ecological restoration performance of the substrate. In previous studies, the algal polysaccharide composite substrate has been preliminarily applied in the ecological restoration of slopes in red soil regions. However, there is still a lack of sufficient research on improving the erosion resistance of the algal polysaccharide composite substrates in red soil. This study aims to address the issues caused by the use of cement as a curing agent in sprayed vegetation techniques, including environmental pollution, difficulty in degradation, and restricted plant growth. [Methods] We selected the red soil slope of a power transmission and transformation project in southern China as the research subject. From previous 16 sets of orthogonal experiments, the best three types of substrates were selected for indoor simulated rainfall experiments. Rainfall intensities were set at 60, 90, 120 mm/h, and the slope gradients were set at 20° and 30°. With a bare slope treated only with clean water as the control group, a total of 72 rainfall experiments were conducted (2 slope angles×3 rainfall intensities×4 slope surfaces×3 repetitions), and the substrate with the best erosion resistance was ultimately selected. [Results] 1) Experimental results showed that on a 20° slope, when the rainfall intensity increased from 60 mm/h to 120 mm/h, the initial runoff yield time for substrates 1, 2, and 3 was shortened by 19.48%, 16.67%, and 30.43%, respectively, compared to the control group. The corresponding stable runoff yield rates were in the ranges of 0.56-2.08, 0.68-2.15,0.61-2.09 L/(min·m²), while the sediment yield rates at rainfall intensities of 60, 90, 120 mm/h were 0.18-0.22 L/(min·m²), 0.41-0.75 L/(min·m²), and 1.12-1.44 g/(min·m²), respectively. On a 30° slope, the initial runoff yield time was shortened by 25.80%, 48.48%, and 40.41%, respectively. The stable runoff yield rates were in the ranges of 0.63-2.21, 0.86-2.38,0.78-2.24 L/(min·m²), and the sediment yield rates under the corresponding rainfall intensities were 0.18-0.39 L/(min·m²), 0.68-0.86 L/(min·m²), and 1.27-1.77 g/(min·m²). 2) The drainage efficiency of the three substrates ranged from 16.35% to 52.73%, with substrate 2 showing the best performance, while the soil conservation efficiency remained stable at 97% to 98%. With increasing rainfall duration, both runoff yield rate and sediment yield rate exhibited a trend of first increasing and then stabilizing. The runoff yield rate of substrate 2 was higher than that of substrates 1 and 3, while the differences in sediment yield rates among the substrates were small. Both the runoff yield rate and sediment yield rate were significantly correlated with runoff shear force (p<0.05). As runoff shear force increased, both the runoff rate and sediment yield rate showed a linear increasing trend. Runoff shear force had the greatest impact on both runoff yield rate and sediment yield, with a significantly higher correlation compared to runoff power, resistance coefficient, and other factors, indicating that it was the primary controlling factor for runoff and sediment yield on the slope. [Conclusion] Algal polysaccharides significantly reduce the initial runoff yield time on substrate slopes, with the reduction becoming more pronounced as the slope angle or rainfall intensity increases. The runoff and sediment yield processes for different substrates under various slope gradients and rainfall intensities are generally consistent. Compared to bare slopes, runoff yield rates of substrate-covered slopes significantly increase, while the sediment yield rates markedly decrease. A significant positive correlation is observed between the runoff yield rate and the hydrodynamic parameters, especially with runoff shear force, which exhibits the highest correlation. However, except for runoff shear force, no significant correlation is found between sediment yield rate and other hydrodynamic parameters. Among the three test substrates, substrate 2 demonstrates superior erosion resistance and is recommended for engineering applications. The optimal formulation for this substrate is algal polysaccharide∶fiber∶fertilizer∶water-retaining agent=1%∶0.8%∶3%∶0.9%. The findings of this study can provide new materials and technical support for similar slope protection projects, promoting the development of green engineering and the high-quality advancement of soil and water conservation.