A high conversion efficiency quasi-optical mode converter prototype is designed for 140GHz TE22,6 applications. The Denisov launcher is designed based on the periodic perturbation concept, leading to primary radiation field with low edge diffraction. Full-vector physical optics integration solver is used to model and analysis the 3-mirror system. And the 3-step iterative phase correction is applied based on the co-polarization field component, so as to achieve high-quality mode conversion. Specifically, the correction of the 1st mirror sufficiently refines the non-ideal radiated fields from the launcher. It is then validated by numerical investigations that, comparing to the original quaritic mirrors, the phase-shaped mirror system leads to excellent conversion performance: The Gaussian content (ηv) of the output fields rises from 92.7% to 99.6%, while the power transmission efficiency (ηp) reaches 98.8%.