Abstract: Abstract: The direct injection (DI) diesel engines are the main power source in modern society. They are widely used in the fields of transportation, construction machinery, agricultural machinery, ships and small machinery. In the face of the challenges of energy saving and environment protection, high-efficient and low-pollution combustion mode has become the development direction of DI diesel engines. The combustion process of DI diesel engines determines the thermal efficiency and the emission levels, while the combustion process is determined by the atomization and mixing process of the fuel. During the operating process of the engine, atomization and mixing of the fuel are controlled by the injection parameters such as the injection pressure and the nozzle diameter of the injector as well as the environmental parameters such as background temperature and environmental density. Therefore, studying the influence of fuel injection parameters and environmental parameters on fuel spray characteristics is of great significance for optimizing the design of combustion system. In this paper, the sensitivity analysis on the effect of background temperatures and densities on the diesel spray characteristics in the previous study was summarized. A direct imaging and schlieren technique of high-speed photography and an image processing program were used to analyze the sensitivities of injection pressure and nozzle diameter to spray parameters. The influence of the injection parameters (injection pressure, nozzle diameter) and ambient parameters (background temperature, background density) on the spray characteristics was compared according to the sensitivity analysis results. The results show that under the experimental conditions (background temperature of 304-770 K, background density of 13-26 kg/m3, nozzle diameter of 0.18-0.26 mm, injection pressure of 120-160 MPa), with the decrease of nozzle diameter, the volume percentage of gas phase tends to increase, and the mean excess air coefficient of the spray also increases. The reason is mainly that as the nozzle diameter decreases, the droplet size decreases, the spray surface area increases, and evaporation becomes faster. With the increase of injection pressure, the volume percentage of gas phase tends to increase, and the mean excess air coefficient of the spray also increases. The reason for this is that with the increase of injection pressure, the speed of oil droplet breaking is faster, the amount of air entrained by the spray is increased, the relative speed between spray and ambient gas increases, and the heat transfer through convection increases, which are beneficial to the evaporation of the spray. It can be found from the sensitivity analysis of gas phase volume percent that the background temperature has the highest sensitivity (3.3) to gas phase volume percent, followed by the injection parameters that can affect the crushing process: nozzle diameter (?0.29) and fuel injection pressure (0.23). The effect of background density on the gas phase volume percent has the lowest sensitivity (0.12). It can be found from the sensitivity analysis of average excess air coefficient that the injection parameters (nozzle diameter (?2.24) and injection pressure (1.29)) have higher sensitivity to the average air excess coefficient, while the environmental parameters (background temperature (0.69) and background density (0.71)) have a slightly lower average effect on the average excess air coefficient.