A detailed theoretical study is made on the amplitude- and phase-frequency characteristics of the static coordinate ramp comparison method with proportional controller and system voltage feedforward， which is one of the currently most widely used control methods. First， the theoretical amplitude and phase difference due to the ramp comparison method are quantitatively derived using differential equations. Then the influence of digital controller delay on the inherent amplitude and phase difference of the current control algorithm is graphically derived using the phasor graph. Finally， an actual amplitude and phase compensation method for ramp comparison current controller is proposed. Experimental results have verified the correctness of the above-mentioned theoretical analysis.