To solve the problem of low interaction efficiency of harmonic gyrotrons, self-consistent theory was used to investigate the dependence of the interaction efficiency on three critical factors; cavity length (quality factor), electronic beam’s velocity ratio and operating voltage. It was found that high efficiency can be achieved by adjusting beam voltage and velocity ratio when the working magnetic field was located at the hard excitation region. Based on the self-consistent nonlinear theory, a W band second harmonic gyrotron oscillator was optimized. Particle in cell computation showed that the efficiency was up to 39.5% when the beam voltage and current was 37kV and 4A, respectively, with velocity spread of 3%.