In order to investigate the development characteristics of multiple injection fuel spray under ultra-high pressure condition, the development characteristics of multiple injection spray for ultra-high pressure fuel supply system were studied by using high-speed photography under high background density. The results show that the longitudinal development of fuel spray is hindered, and fuel accumulation promotes radial development, resulting in the decrease in spray penetration distance(up to 26%) and the increase of spray cone angle(up to 17%) with the increase of background density under the conditions of background temperature 303 K, nozzle diameter 0.148 mm, background density 60-140 kg/m³, and injection pressure 200-250 MPa. Increasing the injection pressure can improve the longitudinal and radial diffusion ability of fuel spray. Due to the pursuit effect between the main and pilot injection fuel, the penetration distance of multiple injection main spray is larger than that of single injection in the range of 10%-30% pilot injection ratio and pilotmain injection interval of 0.4-1.6 ms. With the increase of pilot injection fuel mass and the shortening of pilot-main injection interval time, the pursuit effect strengthens, and the penetration distance of main injection increases(up to 18%). Due to the driving effect of main injection fuel on the pilot injection fuel, the penetration distance of pilot injection fuel spray increases compared with the single injection fuel spray, and the longer the interval time between pilot-main injection, the weaker the driving effect of main injection fuel on the pilot injection fuel. When the interval time is 1.2 ms, the penetration distances of single and pilot injection spray are approximately equal, and the driving effect becomes negligible.