Abstract: In this paper, a bunch length diagnostic model based on RF deflecting cavity, which illustrated the mechanism of how the parameters of incident beam and cavity affect the measurement results, was established, and a bunch length calculation method was proposed. The cavity was modeled by 3D simulation software CST, and the most optimal one-cell cavity size, coupler size and working mode field distribution were obtained. The corresponding time resolution of the deflecting cavity system is 182 fs, which satisfies the design requirement. The beam dynamics simulation was carried out by particle tracking software Parmela, the reason for the transverse deviation of the bunch on the downstream screen was given, and the correctness of the theoretical result was verified.