The maximum efficiency of permanent magnet synchronous motor (PMSM) is studied and analyzed on the problem of the efficiency of motor driving system in the paper. Maximum efficiency and minimal loss have the same meaning, the mathematical model of the inverter and the PMSM are set up, thus the relationship between system loss and the stator flux is derived. Therefore, the value of the maximum efficiency of PMSM driving system can be achieved according to the expression of the stator flux. With the use of MATLAB/SIMULINK, the advantages of the loss model method and the search optimization method can be combined effectively to establish the driving system simulation model. While keeping the output-power constant by detecting the inverter-side DC voltage and current, the maximum efficiency and minimum loss in the overall range can be achieved rapidly.