As a relatively novel technique for drug delivery, the needle-free injection technique is characterized by transporting the drug liquid to the designated subcutaneous position through a high-speed micro-jet. Although this technique has been applied in many fields, the research on its drug dispersion mechanism and injection performance is insufficient. The presented study aims to identify critical parameters during the injection process and describe their influence on the injection effect. The injection completion rate and performance of a needle-free injector under various operating conditions were compared based on mouse experiments. The results show that the nozzle diameter imposes a more significant influence on jet characteristics than other injection parameters. Moreover, the injection completion rate increases with the nozzle diameter. The nozzle diameters of 0.14 mm and 0.25 mm correspond to injection completion rates of 89.7% and 95.8%, respectively. Furthermore, by analyzing the rate of blood glucose change in the tested mice, it is found that insulin administration through the needle-free injection can achieve a drug effect duration longer than 120 min, which is better than that obtained using conventional needle-syringe technique. In summary, the obtained conclusions can provide an important reference for the optimal design and extending application of the air-powered needle-free injector.