The common harmonic gear drive must be input motion and power by the motor and wave generator, which being low space efficiency, large inertia and poor response, therefore, an active harmonic motor driven by giant magnetostrictive material is proposed. Actuator, as the power source and energy conversion device of harmonic motor, has strict requirements for magnetic field intensity, uniformity, magnetic leakage and space restriction. Accordingly, a structure layout of bias magnetic field produced by segmentally built-in permanent magnet is put forward, which could make full use of space and improve uniformity. The main parameters of actuator are designed, and then of further optimization is carried out by analyzing the influence factors on intensity and uniformity of bias and driving magnetic field. Based on the optimized results, the actuator is modeled and manufactured, thereafter, its magnetic field characteristics of actuator are simulated and analyzed by finite element method (FEM), and the output characteristics are verified by experiments. It shows that structure composed of segmentally built-in permanent magnet has few magnetic leakage, moreover, the proposed design theory and method not only guaranteed the magnetic field intensity and uniformity, but also make the actuator working in a linear range, the law of output meeting the requirements of gearing, and easier to control accurately, which lays the foundation for the practical application of giant magnetostrictive harmonic motor. © 2018 Journal of Mechanical Engineering.