Abstract: Backward facing step flow with boundary layer separation and reattachment is a common phenomenon in engineering.The research of backward facing step flow has important theoretical significance and practical value.Backward facing step flow contains a variety of com-plex fluid mechanics,such as flow transition,flow separation,reattachment and unsteady flow. It has important academic significance in the field of fluid mechanics.The test techniques of sur-face pressure and particle image velocimetry have been applied to investigate the backward facing step turbulent separated flow control with synthetic j et.Through analysis of the pressure coeffi-cient distributions of the step wall surface,transient vortex flow structure and time-averaged flow structure,it reveals the separation flow control mechanism of synthetic j et on the backward facing step reattachment length and recirculation zone.The results show that the synthetic j et imposed on the leading edge can effectively reduce the range of the recirculation zone,the vortex structure of which is locked by the synthetic j et perturbation.In this experiment,the greater the momentum coefficient of the synthetic j et is,the better the control effect becomes.The time-av-eraged effect is:when the momentum coefficient of the synthetic jet is 0.771%,the reattachment length can be decreased by 50%.