Based on the theory of mode coupling, 3mm band second-harmonic complex cavity gyrotron inner TE0.3-TE0.2- TE0.1 mode converter and collectors were discussed in detail. By adopting structure of corrugated wavegnide with radius tapered and different phase rematch technique, the reliable optimal geometry parameters were obtained. Corrugated waveguide mode converter of 94GHz gyrotron was designed with compact bulk and high efficiency, and the burned patterns measured at the ends of converter were obtained for the gyrotron.