Abstract: The needle-free injection has broad application prospects in medical fields such as treatment or vaccination prevention. However, there is insufficient understanding of the diffusion dynamics of needle free injection micro jets under the skin, which affects the accuracy of this technology in subcutaneous drug delivery. The pneumatic needle free injector was selected to carry out high-speed photography tests of the gel injection process, and the Smoothed Particle Hydrodynamics method was used to carry out the simulation of the injection process. The diffusion size obtained from the simulation was compared with the observation results. The results indicate that: when the nozzle diameters are of 0.14 mm, 0.25 mm, and 0.32 mm, and the injection dose ranging from 0.1 to 0.3 mL, the maximum diffusion depth L_d increases from 16.3 mm to 27.5 mm with the nozzle diameter increases, representing a change rate exceeding 60%. When the nozzle diameter is fixed while varying the dosage, the diffusion width L_w increases from 12.8 mm to 17.1 mm. It was concluded that the injection dose has a more significant impact on the diffusion width of the drug solution, whereas the nozzle diameter exerts a greater influence on the diffusion depth. Numerical simulations revealed the formation and propagation process of the stress wave as the drug solution jet penetrates into the gel, along with the position-time-stress waveform diagram. These findings validate the conclusion that the diameter of the wound formed during the needle-free injection process corresponds to the nozzle diameter. By adjusting the nozzle diameter and injection volume, precise injection depth and the anticipated diffusion pattern of the drug solution can be achieved. These research outcomes provide a theoretical foundation for optimizing needle-free injection parameters.