Abstract: To address insufficient rotational inertia in power grids with high-penetration renewable energy and transient stability analysis errors from the conventional constant-voltage assumption of virtual synchronous generators (VSG) neglecting reactive power coupling, this paper studies the transient stability mechanism and potential energy function evaluation method for VSGs considering reactive power coupling. First, a VSG large-signal model is built to reveal the intrinsic mechanism that reactive power coupling causes nonlinear distortion of the power-angle curve and shrinks the system stability region. Second, to solve the direct integration difficulty of distorted power-angle curves, the traditional equal-area criterion is improved via constructing a reactive power coupling-aware potential energy primitive function, converting complex area integration into equipotential point root-finding and realizing fast analytical solutions of the maximum swing angle and critical clearing angle. Finally, the effectiveness of the theory and method is verified by a hardware-in-the-loop experimental platform.