In view of the inferior anti-scouring performance of closure gap of sea dyke in silt soft soil beach, we established the basic hydraulic calculation equation for the closure gap of sea dyke based on hydraulics theory and water balance principle. It is a nonlinear ordinary differential equation with precise analytical solution hardly obtained under normal condition. We can firstly obtain the numerical solution using the classic four-order Runge-Kutta method and MATLAB numerical software. In subsequence, we employed the sine function to fit the variation of the ocean tide position, and the quadratic function to fit the relation between storage capacity of closure area and water level. In association with numerical equivalent analysis and calculation experiences, we present the approximate analytic algorithm between the maximum water level difference and the width of closure gap. Engineering case study demonstrated that the numerical algorithm presented in this article meets practical application requirements.