Because of the application of virtual synchronous machine technology, the distributed generation has the characteristic of a synchronous generator, which can achieve off-grid operation. The controllable inertia has great influence on the frequency stability and active power oscillation of the distributed parallel system under the off-grid condition. In this paper, a distributed parallel system of virtual synchronous machine under off-grid condition is presented. The output active power oscillation at both ends are the standard. The Lyapunov method is used to analyze the influence of inertia time constant at both ends on the active power oscillation of the system. Then based on the analysis result, the inertial time constant is controlled to reduce active power oscillation range and time during the transient period, so as to improve the transient stability of the system. At last, a simulation model is established in PSCAD/EMTDC? to verify the feasibility of the proposed method. This work is supported by National Natural Science Foundation of China (No. 51677066).