At present, the discrimination of the submergence limit and the submergence coefficient calculating method of the low practical weir sluice discharge with polygonal line are not yet mature and perfect.Based on a hydraulic model tests on the discharge of the low weir with the weir height T≤2.0m and with different combinations of T/δ(whereδ is the weir crest length), the submergence limit discrimination and submergence coefficient calculation method of the discharge were carried out, and corresponding discrimination criteria and calculation methods were proposed.The main achievements are:①The critical submergence limit of the low weir with polygonal line is smaller than the limit of the broad-crested weir flow, and decreases with the increase of T and T/δ;②Under the same submergence degree (h_s/H₀), the submergence coefficientσof the low weir is smaller than that of the broad-crested weir, and the relative errorη between the two types of weir increases with the increase of T, T/δ,and (h_s/H₀);③Under the condition of (h_s/H₀)≤0.96, the calculation method for the correction factor of the submergence coefficient σ of the weir is obtained. The research results have been validated by relevant engineering hydraulic model test results, which can provide reference for similar engineering design.