According to the problem of drawing force control in the fused biconical tapering process of polarization maintaining fiber (PMF) coupler, an online computer control method of drawing force is presented, which includes the computer, drawing mechanism, rotary position encoders and a control circuit. By detecting the rotation angle of fiber clamp, the electro-magnetic force can be controlled through changing the coil current, and the real-time control of drawing force is realized. Firstly, the relationship between the electromagnetic force and drawing force is analyzed, a 3D finite element model of the permanent magnet and the coil is established, and a trajectory equation describing the permanent magnet is obtained. Then, by simulation of the magnetic field and the electro-magnetic force around the coil, the relationship among the coil current, the rotation angle of fiber clamp and the drawing force is determined. At last, by simulation, the drawing force is controlled within an error of 1.03%.