Aiming at the shortcomings that the single injection dose of the current single-hole needle-free injector is small and requires repeated injections， a large-dose multi-hole needle-free injector is proposed. ANSYS Workbench simulation platform was used to analyze the flow field of the needle-free injector， and the working pressure range was gained not less than 13 MPa. Meanwhile， the stress variation law of the needle-free injector under different diameters was obtained by fluid-structure interaction analysis. The relationship polynomial between the allowable stress and the minimum ampoule diameter under various pressures is fitted. According to the simulation results， an orthogonal test was designed to optimize the structure of the contraction section for the needle-free injector. The optimized scheme was obtained as follows： the contraction angle was 20°， the aspect ratio was 1.4， the diameter of the distribution circle was 2 mm， the contraction section length was 4.05 mm， and the diameter of the micro-hole was 0.165 mm. Through the stress analysis of the ampoule， it is verified that the optimized scheme can meet the mechanical property requirements of the material. The optimization results show that a larger dose is achieved than the traditional single-hole injector， the dose can be up to 5 mL， and the injection speed is as high as 150 m/s， which can penetrate the skin without causing too much damage. The feasibility of the optimization scheme was further verified through experiments and kinetic analysis.