Wave transmission coefficient formula in Code of Design for Breakwater and Revetment only takes relative depth above the submerged breakwater crown into consideration. Based on numerical simulation using MIKE21 SW model，we carry out a study on the factors of wave transmission coefficient and analyze the suitability of the formula under different conditions. Results show that relative depth and relative width of the submerged breakwater crown are major factors. When 2≤B/H≤3 and hc/H≤1.25，the formula works well；When B/H≤1 or B/H≥4 or 2.0＞hc/H≥1.5，it is suggested to carry out experimentation to estimate the wave transmission coefficient.