This paper expounds the superiority of adopting four-step operational mode for TGP lock under impounded level 156 m in the finishing construction period, and demonstrates the feasibility of adopting four-step operational mode with the hydraulic conditions. After adopting the optimizing water conveyance pattern by means of prototype debug, the cavitation of the 1st-bifurcation at mid chamber has been basically eliminated, the working conditions of the lock under exceeding design water head 45.2 m has been confirmed well, therefore, the four-step operational mode can be adopted for TGP lock.