Based on the coupled-wave theory and the phase-matching technique, an improved method for low-order quasi-optical mode launcher is presented. Adjusting the phase difference between the working mode and its satellite modes, an ideal mixture mode with the features of Gaussian field distribution was obtained. Using the proposed method, a 94 GHz TE5,3 quasi-optical mode launcher was synthesized. The numerical calculation shows that the difference between the edge and peak values of field intensity reaches 30 dB. A well-focused field at the aperture with a Gaussian mode content of more than 98% can be obtained，which will reduce drastically the diffraction losses and reflection of power from the cuts.