This paper develops a control strategy for tractor semi-trailers by active trailer steering, aiming at minimizing the sweep width of the vehicle at low speeds. The metric of sweep width is defined to evaluate the maneuverability of tractor semi-trailers under a circular motion with constant speeds. The active steering angle of the trailer for each given front wheel steering angle of the tractor is determined to minimize the sweep width, based on solution of equilibrium equations of both the tractor and the trailer in the yaw plane at a very low speed such as 5 km/h. The two steering angles of the tractor and the trailer are fitted to form an open-loop active-steering control algorithm. A nonlinear tractor semi-trailer system model is built for co-simulation purpose by using TruckSim and MATLAB/SIMULINK to evaluate the active trailer steering algorithm. Simulation results show that, under the low-speed roundabout and double lane change maneuvers, the active trailer steering controller can significantly reduce the sweep width and thus improve the low-speed maneuverability.