An electromagnetic force device is designed to realize the precise force control in the process of fusion stretching of ultra-thin fibers. First, the electromagnetic force with different coil parameters is calculated based on finite element method, and the relationship between electromagnetic force and coil parameters is derived. With the demand of stretching system performance and the dimension restrictions of the coil frame, constraint equations and objective functions are established to optimize the solution to machine the electromagnetic coil. Then the control circuit for electromagnetic force is designed to regulate the current in the coil and achieve the fine adjustment of electromagnetic force. Finally, experiments of electromagnetic force device are performed. The results indicate that the range of stretching force for the optical fibers can achieve 26.073 mN and the measuring resolution is 7.473 μN. It satisfies the demand of the fusion stretching of ultra-thin fibers.