Abstract: Phase plays a key role in the design of gyrotron quasi-optical mode converters because it provides important information about the electromagnetic field distribution. In this paper, a 170 GHz, TE_31,8 mode gyrotron QOMC design has been completed around phase correction. A hybrid launcher has been designed based on the scalar diffraction integral theory and the fast Fourier transform (FFT) method. The structure supports arbitrary forms of internal wall perturbation and achieves a vectorial Gaussian content of 98.7% at the radial aperture. In addition, a three-stage mirror system is designed using a phase difference algorithm in conjunction with Gaussian beam propagation theory. The lateral output efficiency and Gaussian mode purity of the beam are effectively improved. The results are also analyzed in comparison with the traditional Katsenelenbaum-Semenov(KS) algorithm. The simulation results show that the phase difference algorithm correction is more effective. The final beam has a scalar Gaussian content of up to 99.6% and a vectorial Gaussian content of up to 98.2% on the output window.