This paper uses the approaches and models of game theory to analyze principal-agent problem between fund manager and fund firm. We analyze the asymmetric information between fund manager and fund firm. We describe the income and cost at this principal-agent problem under asymmetric information, give mathematics expressions about every step action decision and their result-the cost, the profit of this principal-agent problem. We present a game model based on principal-agent theory under the asymmetric information. Then, discuss the optimum principal-agent cost, the profit and the action between fund manager and fund firm based on our model. According to our model, a conclusion can be drew that the effort degree under asymmetric information is less than perfect information. Asymmetric information between fund manager and fund firm will lead to moral hazard. The lower effort level will lead to the agent cost which does not exist in perfect information circumstance. The intensity of fund firm selected is due to the supervise cost and correlative profit.