In next generation mobile networks, more and more throughput of network is required under the dense environment. There are some researches discuss how to solve the predicament of throughput. But they usually analyze it under the saturated environment. Most of their contention window size are also not optimal to fetch the maximum throughput. This paper is the first one to fetch the fixed optimal window size under the unsaturated environment which is closer to the real network. It is based on the Distributed Coordination Function (DCF) in Medium Access Control layer of IEEE 802.11 Wireless LAN to control the contention window size for reducing the collision problem and improving the throughput. A simplified one-dimension Markov Chain with a new idle state is proposed to simulate the unsaturated model. Then the formula of transmission probability under the unsaturated model and the equation which is used to fetch the optimized contention window size are analyzed. Comparing with the related analysis model in the simulation of the throughput and the collision rate. The result shows that the proposed model under unsaturated environment is better than before.