Terahertz Traveling Wave Tube(TWT) cascade multiplier is proposed for a compact source of high-power, wide-band THz radiation as a novel vacuum electronics device. For THz applications, the design and feasibility studies of a multiplier concept are presented by using a TWT with cascade harmonic system to satisfy the requirements of harmonic information in spent beam of TWT. Using Microwave Tube Simulator Suite(MTSS), the information of W-band TWT multiplier with cascade 2nd harmonic system indicates that this reasonable and practical device has good performance compared with other compact source operated in the frequency region from 140 GHz to 220 GHz. The harmonic power reaches above 2 W in 8 GHz with 37 dB conversion gain. 3-D PIC beam-wave interaction is simulated using CST Particle Studio which shows that THz TWT cascade multiplier bears good power and bandwidth performance and practicability as potential THz radiation source.