To quantitatively study relationship between the transferred power and the AC output voltage of STATCOM, the method based on the sensitivity analysis is utilized. The voltage phasor equation of STATCOM is established based on the KVL, and two independent equations are acquired according to the real component and the imaginary component, respectively. By calculating partial derivatives of the RMS value and the phase angle of STATCOM AC output voltage to the active power and the reactive power, corresponding sensitivity results are obtained. It shows that the influence of the reactive power on the RMS value and the phase angle of AC output voltage is much greater than that of the active power. Moreover, the AC output voltages of STATCOM calculated by the direct method are compared with those by the sensitivity method, verifying the correctness and accuracy of the presented sensitivity method. This work is supported by National Natural Science Foundation of China (No. 51677092).