The security of cyber-physical systems (CPS) has become an active research area in recent years. Autonomous micro-grid, an important component of smart grid and essentially a cyber-physical system, extends the attack surface and undermines system security. This paper proposes a game-theoretic approach to describe the plausible attack-defense dynamics for autonomous micro-grid. The game is a kind of two-player, one shot, non-cooperative, and Zero-sum game, where the goal of attackers in this game is to gain the maximum payoff under a certain cost. The behavior of attackers includes two steps, intrusion and disruption, whereas the defenders can improve their defense performance by updation or reinforcement of cyber component. The optimal strategy of attackers is deduced based on the dynamic model. Finally, the case studies validate the proposed method and demonstrate its feasibility.