Abstract: Since the structure of flow boundary layer can be modified by the electro-magnetic body forces, one of active control methods might be developed by adjusting the forces so as to control the layer. Based on the equations of magnetic and electric fields and the formula of the flow, the distribution of Lorentz force and its control effect on cylinder wake flow have been investigated numerically in the electro-magnetic fields formed by a moving low-conducting electrolyte. The influences of actuator width to the distribution of electro-magnetic fields, Lorentz force, flow field are simulated and discussed. The electro-magnetic fields are covered from the separation points to the end of cylinder. While the interaction parameter N is small, the separation points on the cylinder surface will be moved rearward, but it can not be suppressed completely. In this case, our simulated results show the better control effect. With the increase of N reaching to the critical value, since the narrow actuator has a large surface vortex, its suppressing effect for flow separation is prominent. When N becomes large, the separation points can be suppressed completely, and the suppression effect is independent of the actuator width.