Abstract: Electron gun is one of the key components of coupled-cavity traveling-wave tube (TWT). Its electron beam parameters have important influence on TWT performances. As a result, electron gun was firstly simulated when an S-band coupled-cavity TWT was designed using PIC software. Then, the three-dimensional simulationmodel of whole TWT was constructed, which consists of electron gun, slow-wave structure, microwave input and output coupler, etc. Finally, the large signal beam-wave interaction of the whole S-band TWT was simulated. Results show that the continuous wave output power reaches 5.9 kW and the efficiency reaches 24% in the operating frequency range of 2.0-2.3 GHz. Whole-tube simulation provides reference for TWT design.