When series capacitor compensation (that is the series compensation) equipment is applied to a high-voltage transmission line, it increases the possibility of maloperation or a miss trip of current differential protection. This paper summarizes the principle and constitution of traditional current differential protection, and the deficiency of traditional braking characteristics. The effect of series compensation equipment on traditional current differential protection is analyzed, and this paper puts forward a protection criterion based on amplitude and phase characteristics. This criterion makes use of the phase angle difference and the minimum amplitude ratio of the terminal current (or fault component current) of a transmission line with its series complement to form the current amplitude and phase plane, and through the analysis of the four stages of the current reverse region caused by the series complement equipment, the current composition of the current differential protection is formulated. Then the current differential protection criterion is further formed. The effects of series compensation equipment, transition resistance, and capacitive current are considered in the simulation. The simulations verify the effectiveness of the protection. This work is supported by the National Natural Science Foundation of China (No. 51807109) and the Key Project of Scientific and Technological Research Program of Hubei Education Department (No. D20171203).