A nonlinear model for the numerical simulation of coupled-cavity traveling wave tube is described. The model is based on the Curnow equivalent circuit from which the matrix equations for the fields of any single cavity can be obtained. And the model can be divided into the matrix equations, the motion equations and the space-charge field equations, in which the sever, dynamic tapering in the circuit and the multiple-frequency signal amplification can be also considered. The simulation is used to analyze the nonlinear AM-PM distortion and the intermodulation products of a coupled cavity TWT whose working frequencies range from 59 GHz to 64 GHz. And the analysis of the saturated output power within the frequency band is also included. Comparing the simulation results with the measurements, the percentage difference between the calculation results and the test results is less than 5%.