Taking into account the relatively low efficiency of brushless doubly fed induction generator (BDFIG) at present, the efficiency optimization is important to promote the industrial application of BDFIGs. The power winding (PW) frequency of the BDFIG-dc system is with the additional freedom of control, which provides an important condition for efficiency optimization. In this article, based on the loss analysis of the power converter and the improved steady-state equivalent circuit of BDFIG considering iron loss, the loss model of BDFIG-dc system is derived. Furthermore, the system loss model is established with the PW frequency of BDFIG as the main state variable. From the proposed loss model, the maximum efficiency of BDFIG-dc system can be achieved by optimizing the PW frequency. The comprehensive simulation and experimental results demonstrate that proposed efficiency optimization control strategy can increase the efficiency of both the standalone and grid-connected BDFIG-dc systems by 0.2-7.8% under different operation conditions.