[Objective] Pile-soil stress ratio is a key parameter in the design of rigid pile composite foundations for shallow-buried rock and soft soil foundations, but the rules governing its value and influencing factors remain unclear. [Method] Based on a concrete sluice dam project, this study carried out 7 sets of scaled indoor physical model tests, systematically studied the bearing characteristics of soft soil single-pile composite foundations and the pile-soil stress ratio, analyzed the influences of factors such as cushion type, pile loading conditions, pile spacing and pile end bearing stratum, and obtained the variation trend of pile-soil stress ratio with load and the pile-soil stress ratio corresponding to bearing capacity. [Results] In the initial loading stage, the P-S curves of the tested single-pile composite foundations exhibited linear changes, and the soil under the bearing plate was in an elastic deformation state. With the increase of load, a sudden change in slope appeared in the P-S curve of the bearing pile at the lower end of the cement-soil cushion, and the characteristic value of foundation bearing capacity should be inferred according to the proportional limit. The P-S curves of friction piles under the cement-soil cushion and friction piles and end-bearing piles under the gravel cushion mainly exhibited a gradual change characteristic, and the characteristic value of foundation bearing capacity should be estimated according to the relative deformation value of 1% of the side length of the bearing plate. Under the two cushion conditions, the characteristic values of the bearing capacity of the single-pile composite foundation of end-bearing piles could meet the design requirements, while those of friction piles could not. The pile-soil stress ratio of end-bearing piles basically showed a monotonous increase with load, while that of friction piles showed an initial increase followed by a decrease. The maximum pile-soil stress ratio of end-bearing piles in the cement-soil cushion was 16.8, and that of friction piles was 13.7, with an increase of about 22.6% for end-bearing piles. The pile-soil stress ratio of end-bearing piles corresponding to the design bearing capacity of 290 kPa could be taken as 9.7, and that of friction piles could be taken as 8.1. Therefore, for shallow buried rock-soft soil foundations, rigid pile composite foundations should adopt end-bearing piles embedded in rock. The bearing capacity of end-bearing piles in the cement-soil cushion was close to that in the gravel cushion, but the pile-soil stress ratio decreased from 16.8 to 8.2, a decrease of about 51.2%, indicating that the gravel cushion had a better stress adjustment capacity. The pile end bearing stratum and pile spacing were key design parameters. When the pile spacing was adjusted from 1.4 m to 1.8 m, and the pile tip bearing stratum was adjusted from weakly weathered rock to strongly weathered rock, the pile-soil stress ratio decreased by 42.3%, but the bearing capacity still met the design requirements, which was more economical. [Conclusion] The cement-soil cushion significantly improves the bearing capacity of the composite foundation and the pile-soil stress ratio, and its maximum pile-soil stress ratio is about twice that of the gravel cushion. The strength of the pile end bearing stratum and the pile spacing have a significant influence on the pile-soil stress ratio, and the scheme can be optimized by increasing the pile spacing and adjusting the pile end bearing stratum. The bearing capacity of the end-bearing pile composite foundation under the cement-soil cushion is close to that under the gravel cushion, but the pile-soil stress ratio is higher. A composite embedded cushion layer can meet both seepage control and stress adjustment requirements. The research results can provide a theoretical basis for the optimal design of rigid pile composite foundations in shallow buried rock-soft soils.