Series-series (SS) compensation is widely adopted in magnetic resonance wireless power transfer (WPT) systems due to its lowest component counts and high efficiency. However, the most undesirable feature of the SS compensation is that its primary and secondary coil current as well as the output current increase rapidly when the coupling between the primary and the secondary coil decreases. The converter and the resonator can be easily damaged due to the high current stress. In this article, a reliable constant output current control method based on primary coil peak current detection is proposed. With the proposed method, constant output current can be achieved within a certain coupling range and the high current stress can be prevented under extremely weak coupling condition.