The theoretical study of the water depth after its jump is not yet mature and perfect. According to the experimental data of the existing literatures，the assumptions on the average water depth in the backflow zone in the abrupt-expanding hydraulic jump are made in this paper. The theoretical equations for calculating the conjugate water depth of the hydraulic jump with abrupt expansion are deduced by the momentum theorem and determines the relevant parameters by using experimental data. The calculation equation with high accuracy are obtained. Comparing the results of several existing conjugate water depth equations with the experimental results，it shows that the equation in this paper has higher accuracy and the calculated water depth after the jump is more in line with the actual situation.