In this paper, we first introduce the reasons of co-channel interference in WLAN, and then build a dense WLAN environment in NS2. We explore the strength of the co-channel interference, as the number of APs increase in a dense WLAN environment, by measuring its total throughput, end to end delay and the average throughput. The simulation results indicate that the strength of co-channel interference follow certain rules. It provides a basis for controlling the strength of co-channel interference when we plan the WLAN network. It also provides references for the simulation study of the dense WLAN.