Liulaojian multiline-lock is located at Beijing-Hangzhou Grand Canal. To solve the problem that when the No.2 and No.3 lock to drain, the No.1 lock reverse waterhead is too high to make the lower miter gate hoist mechanism damage, a two-dimensional flow mathematical model is developed to calculate the upstream and downstream flow condition of the ship lock, in accordance with the Code for Master Design of Shiplocks, compare the navigation flow condition of various scheme, and choose the engineering measures which is satisfied the demands. Due to the complex characteristics of the natural river′s boundary, we adopt the unstructured triangular mesh and use the finite volume method to discrete the flow control equation.