Once DC with high amplitude penetrates the power system through the neutral point of the transformer, line CT will saturate to some extent. This saturation will affect the measurement and control of the secondary system. When the DC bias is very serious, it will even induce an incorrect relay protection of the system. This poses a serious threat to the safe and stable operation of the power system. Therefore, an improved scheme of relay protection based on phase space trajectory identification is proposed. First, the adverse effect of DC bias on protective CT is analyzed. Then, taking line protection and transformer protection as examples, the failure mechanism of relay protection under the influence of DC bias is revealed. An improved relay protection scheme based on phase space trajectory identification is proposed, one which can effectively deal with the protection failure problem caused by CT saturation, based on the characteristic that the phase space trajectory is not affected by time-domain saturation. Finally, taking a typical protection failure scenario as an example, the effectiveness and superiority of the proposed scheme are verified by comparing the protection actions of the traditional method and the proposed scheme. This work is supported by the General Program of National Natural Science Foundation of China (No. 51877088).