The dq coordinate impedance model of current-controlled parallel inverter is established, and the influence of system frequency disturbance is introduced. Based on the impedance modeling of virtual synchronous generator, the dq coordinates impedance model of microgrid synchronous generator is established. The multi-impedance criterion which is more suitable for power electronic systems is used to analyze the instability problems that may occur during the process of load change when multiple parallel inverters and synchronous generators in parallel operation. Through this method, the process of judging the stability of system is simplified. Finally, the simulation verifies the correctness of the theory.