A high conversion efficiency quasi-optical mode converter prototype is designed for 140 GHz TE_22，6 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 analyze the 3-mirror system. And the 3-mirror 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 1^st 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 （η_v） of the output fields rises from 92.7% to 99.6%， while the power transmission efficiency （η_p） reaches 98.8%.