The virtual synchronous machine can improve the inertia of the new power system, but the current virtual synchronous machine model mostly adopts the sequential impedance model, and the dq impedance model is not perfect, because the dq impedance model has better effect for the stability analysis and the control analysis, therefore, this paper establishes a detailed dq model of the virtual synchronous machine, and this model considers the power decoupling, which facilitates the power coupling analysis, and it has better accuracy. Then Nyquist curve is used to evaluate the effect of decoupling method on the model. Finally, the validity of the established model is verified by theoretical analysis and experimental results.