By analyzing the throughput model of IEEE 802.11 MAC DCF put forward by Bianchi G, the retransmitting limit is extended and the state of direct transmission with no backing off is added in its discrete Markov chains to refine and perfect the classic model. The following assumption is proposed: the number of wireless stations accessing to AP in BSS met poisson distribution within a certain period of time. Under this assumption, both of the corresponding model and the quantified expression of calculating its goodput are provided. The rationality and effectiveness of the proposed model are verified with emulation of unidirectional TCP flow from AP to wireless stations.