Gyrotron traveling wave amplifier is a very important high power millimeter wave source for radar applications. Second-harmonic operation is of special significance due to its low magnetic field operation. By using an interaction circuit with two-stage distributed-loss, the absolute instability and the gyrotron backward wave oscillation can be suppressed. Nonlinear analysis is used to study the stability and large signal performance of the amplifier. Particle in cell simulation of W-band TE02 gyrotron traveling wave amplifier are also performed. The results show that this amplifier can generate 300 kW at 94 GHz with 15% efficiency and 5% bandwidth in case of a 100 kV, 20 A electron beam input.