[Objective] At present, the calculation methods for the discharge capacity of polygonal-line low-head practical weirs at sluices (such as the discrimination of the submergence limit and the calculation of the submergence coefficient) are still incomplete, which brings considerable inconvenience to related engineering design. This study aims to investigate the submergence limit discrimination and submergence coefficient calculation of polygonal-line low-head practical weir flow, and to propose corresponding discrimination criteria and calculation methods. [Methods] Hydraulic normal model tests on the discharge capacity of polygonal-line low-head practical weirs at sluices were conducted. The model conditions included a weir height T≤2.0 m (T=0, 0.5, 1.0, 1.5, and 2.0 m), a ratio of weir height to crest length T/δ≤0.4, and a submergence ratio h_s/H₀≤0.96, where h_s was the downstream water depth referenced to the weir crest, and H₀ was the total upstream head above the weir crest. The model scale was 35.5. [Results] (1) Under the experimental conditions of this study, the critical submergence limit (h_s/H₀) of the weir flow ranged from 0.748 to 0.787, which was lower than that of the broad-crested weir (0.8), indicating that the polygonal-line low-head practical weir at a sluice was more susceptible to submergence than a broad-crested weir. As the weir height T and T/δ increased, the critical submergence limit decreased correspondingly, and the difference between the critical submergence limit of the polygonal-line weir and that of the broad-crested weir increased accordingly. (2) Under the same submergence ratio h_s/H₀, the submergence coefficient σ of the polygonal-line low-head practical weir was smaller than that of the broad-crested weir σ₀. The absolute value of the relative error η between the two increased with increasing T, T/δ, and h_s/H₀. Compared with the geometric parameters of the polygonal-line low-head practical weir, namely the weir height T and the crest length δ, the variations in weir height T had a relatively greater influence on the relative error η. (3) When h_s/H₀ ranged from 0.8 to 0.96, the relative error η ranged from -1.5% to -7.6%. (4) Based on the weir height T and submergence ratio h_s/H₀, the relative error η can be obtained from the η-T-h_s/H₀ relationships shown in Table 4 and Figure 8. The corresponding submergence coefficient of the polygonal-line low-head practical weir could then be calculated using the formula and applied to discharge capacity calculation. (5) The results were validated by multiple hydraulic model test results of sluice projects. The discharge values calculated based on the present results were in good agreement with the measured values, with the absolute value of the relative error between the two being less than 1.5%. [Conclusion] The findings of this study can be applied to the calculation of discharge capacity of polygonal-line low-head practical weirs at sluices under the conditions of T≤2.0 m, T/δ≤0.4, and h_s/H₀≤0.96. Further in-depth studies are still required on the discharge characteristics of polygonal-line low-head practical weirs with T>2.0 m and larger ranges of T/δ. The proposed method has relatively high calculation accuracy and convenient application, and can provide a reference for the design and operation of similar projects.