The expansion of wireless broadband access network deployments is resulting in increased scarcity of available radio spectrum. To expand LTE capacity

increase the service efficiency of spectrum

LAA deploy the LTE system in the unlicensed bands

coexist with other wireless access technologies（such as Wi-Fi）

which has become the focus of the research. Hence

It is the heart of the matter that how to design a reasonable competition mechanism on LAA system. This paper mainly studies the coexistence mechanism of running LTE and Wi-Fi system simultaneously in the unlicensed frequency bands and achieves the system-level simulation. Three kinds of back-off algorithms have been put forward and optimized.