To study the cavitation occurring in diesel nozzle and its effects on the flow capacity, the process of cavitation and the flow characteristics in diesel injector orifice were analyzed based on optical testing and threedimensional CFD simulation. In the experiment, the visualization of cavitation was recorded by a highspeed camera, the frame rate was set to 6688 frames per second. The highfrequency Kistler pressure transducers were assembled before and after the nozzle to acquire pressure. Ultimately, the data of pictures and pressure were stored by a computer. The 3D CFD diesel cavitation simulations were carried out under conditions of different inlet pressures and back pressures based on the commercial code Fluent. The CFD simulations were validated by the experimental data. The results showed that: when K>2.49, there were no bubbles in the nozzle, which was named nocavitation period; during nocavitation period, both the gasliquid mixed phase volume fraction and the fuel mass flow in each section did not increase and the flow coefficient was constant. When K≤2.49, it is the cavitation developing period when the gasliquid mixed phase volume fraction of the entrance of the nozzle (plane A) started to increase which represented cavitation inception. At the same time, the growth rate of fuel mass flow reduced, and the flow coefficient began to decreased. When K≤1.91, the gasliquid mixed phase volume at plane A became stable, which meant the cavitation saturation period. During the saturation period, the fuel mass flow rised to its maximum and maintained stable while the flow coefficient decreased. It can be seen that the cavitation occurred in diesel injector nozzle had large impact on the flow characteristics. Meanwhile, the cavitation had impact on the fuel mass flow, the flow coefficient and the gasliquid mixing phase volume fraction at different nozzle cross sections.