Abstract: In the low-temperature environment of diesel fuel viscosity increases fluidity deterioration, inhibiting the diesel engine diesel spray atomization and evaporation, directly affecting the reliability of the diesel engine cold start. For this reason, the backlight method test was used to compare and study the change rule of spray macrostructure of high temperature and low temperature diesel under different working conditions. The digital image processing of the spray image was carried out by using MATLAB to obtain the macroscopic characteristic parameters of the spray. The test results showed that when the diameter of the spray hole 0.12 mm with the change of injection pressure diesel temperature on the injection volume of the impact was more obvious. The continuous viscous force slowed down the flow velocity of the fluid layer near the wall to suppress the diesel emission. When the diesel temperature was lower than 0 ℃, the decrease in diesel temperature led to a reduction in the amount of injection. At high injection pressure, the decrease of diesel temperature led to the enhanced viscous force between droplets, which promoted the continuous ejection of diesel under the action of kinetic energy and inertial force, and the fuel injection amount increased with the decrease of diesel temperature. When the injection pressure was 75 MPa, the diesel temperature dropped to -20 ℃ compared with 38 ℃ diesel injection quantity increased by nearly 23.87%. And the decrease of diesel temperature under small pore size led to the increase of viscous force between droplets, which led to the increase of droplet size and the greater momentum of droplets. Thus, the radial development trend of diesel spray was weakened and the axial movement ability was enhanced. With the decrease of diesel temperature, the spray penetration increased and the spray cone angle decreased. The diesel spray penetration increased by 39.89 mm at 0.70 ms when the diesel temperature decreased from 38 ℃ to -20 ℃. When the nozzle diameter was 0.28 mm, the injection pressure was less than 75 MPa, and the injection quantity decreased with the decrease of diesel temperature. Under the same injection pressure, more momentum of diesel was consumed to overcome greater internal friction, so that its ability to reach the farthest distance was weakened. Reducing diesel temperature led to a decrease in spray penetration and spray cone angle. When the injection pressure is 75 MPa, the diesel spray penetration at- 20 ℃ at 0.70 ms is 30.86 mm shorter than that at 38 ℃. Increasing the nozzle diameter at the same injection pressure, the spray penetration of -20 ℃ diesel decreased and the spray cone angle increased. Secondly, with the increase of injection pressure, the influence of diesel temperature on the macroscopic characteristics of spray was more prominent. The higher the injection pressure, the more obvious the shortening trend of spray penetration caused by the decrease of diesel temperature. Therefore, appropriately increasing the injection pressure and increasing the nozzle diameter were beneficial to the evaporation and gasification of low temperature diesel liquid phase spray.