In order to improve the dynamic response characteristics of existing injectors, a design scheme of giant magnetostrictive injector (GMI) was proposed by using a giant magnetostrictive actuator (GMA) to drive a ball valve through a hydraulic chamber. Based on the structure and working principle of the injector, the output force model of the GMA was established based on the Jiles-Atherton hysteresis model, which was coupled with the hydraulic model of the injector, and the electro-magnetic-mechanical-hydraulic multi-field coupling model of the injector was established. The simulation model of the injector was built based on MATLAB/Simulink module, and the magnetostrictive curves of GMA under different bias magnetic fields and the response curves of ball valve under different GMA prestresses were analyzed. The key parameters of GMA, such as preload, inlet and outlet hole diameter and control chamber volume, were optimized by genetic algorithm and needle valve response time was taken as evaluation index. The results show that the designed GMI has good response characteristics, the opening delay of the needle valve is reduced by 27.3%, the rise time of the needle valve is reduced by 11.0%, the closing delay of the needle valve is reduced by 19.5%, and the drop time of the needle valve is reduced by 9.9%.