For loosely coupled wireless power transfer systems, strongly magnetic coupling is becoming a viable scheme to realize power transfer over medium distances. The method using DC-DC circuit is regarded as the most promising way to realize impedance matching for such systems. However, due to the nonlinear nature of rectifying circuit, it is difficult to track the optimal load accurately and the robust stability can not be guaranteed. Based on the above considerations, one tracking operator-based optimal load control method is proposed in this paper. The proposed control system can track the optimal load with high accuracy even when the output load varies, the tracking performance and stability can be verified. Moreover, the robust stability is considered using operator theory. Finally, simulation results are presented to confirm the effectiveness of the proposed control scheme.