Staggered double vane slow wave structure (SWS) and sheet electron beam are employed to investigate a 140 GHz high power traveling wave tube. Numerical calculation of eigenmode shows that the SWS has a good characteristic of dispersion and interaction impedance. The transition structure, input/output coupler and concentrated attenuator are especially proposed for the circuit to ensure that the tube will work well. Particle-in-cell simulation results demonstrate that the traveling wave tube can provide over 300 W of peak power in a frequency range of 132-152 GHz with a maximum of 546 W and a corresponding gain of 37.37 dB at 138 GHz assuming a beam power to be 5.115 kW and input power to be 0.1 W. The output power of the tube can exceed 440 W in a frequency range of 128-152 GHz with a corresponding interaction efficiency of over 8.6% when the input powers range from 0.027 W to 0.46 W. Such a traveling wave tube has a great significance and a potential application in high power short millimeter wave field.