Taking into account the relatively low efficiency of brushless doubly-fed induction generator (BDFIG) at present, the efficiency optimization is very important to promote the industrial application of BDFIGs. Based on the improved steady-state equivalent circuit considering the iron loss, this paper analyzes the relationship between the efficiency of BDFIG and power winding (PW) voltage frequency, from which the optimal PW frequency for maximum efficiency can be achieved. This means that the operation mode of BDFIG can be changed from the constant-frequency-constant-voltage (CFCV) mode to the variable-frequency-constant-voltage (VFCV) mode in order to obtain the maximum efficiency. The efficiency optimization method based on the VFCV mode can be applied to BDFIG DC systems for the ship-shaft and wind power generation, since the PW frequency of the BDFIG-DC systems is with the additional freedom of control. The simulation and the experimental results verify the proposed method.