The lattice Boltzmann method (LBM) is a mesoscopic numerical simulation method developed in recent decades which has good ability of dealing with complex boundary and parallel computing.The simulations were performed for back facing step flow by LBM, and the characteristics of the back step flow under different Reynolds numbers were obtained. The research results showed that, in the case of low Reynolds number, the distance between the reattachment point and the separation point became larger as the Reynolds number increased, the value of K increased with the increase of Reynolds number, while the growth rate gradually became smaller. In the case of high Reynolds number, the distance between the reattachment point and the separation point increased with the increase of the Reynolds number, and the growth rate gradually decreased. When Re=8000, the distance between the reattachment point and the separation point tended to be stable. The value of K first became larger as the Reynolds number increased, and the increase rate became smaller as the Reynolds number increased, and eventually tended to be stable. For the back step flow under high Reynolds number, the simulation was performed by the LES⁃LBM method, which was in good agreement with the previous experimental data. It showed that the LBM⁃LES model had high accuracy for simulating the back facing step flow after high Reynolds number.