This paper presents a converter impedance calculation method under virtual synchronous machine (VSM) control. Firstly, the conventional virtual synchronous machine control strategy is presented, in which the active and reactive power control is realized. Secondly, the small signal model with VSM control is deduced, thus a calculation method for impedance in d-q frame is proposed. The effect produced by voltage and frequency adjustment in VSM control to d-q frame voltage is fully analyzed. This method also extends to capacitor voltage close loop control for static var compensation field. At last, the calculated impedance is compared with the measured impedance using Matlab/Simulink. Results validate the correctness of the proposed impedance calculation method.