Due to safety and convenience, aviation heavy fuel (AHF) is quite suitable for use as an energy source in aviation piston engines for small aerial drones, although its atomization is an important issue. The purpose of this article is to present the atomization mechanism of AHF during low-pressure direct injection (LPDI) and the results of the investigation of the mixing process and flow state of fuel–air two-phase flows. In this study, experimental data were obtained for parameters of fuel spray, which verified the improved calculation model of LPDI that considered the primary atomization of AHF inside the premixing chamber. The influences of injection pressure, ambient pressure, and AHF temperature on the spray characteristics were compared and analyzed. Increasing the injection pressure reduced the spray cone angle and increased the spray area. The penetration distance increased, and the Sauter mean diameter (SMD) of the fuel droplets decreased. Increasing the ambient pressure had significant effects on penetration distance and SMD. The spray area decreased, and the spray cone angle showed small variations. Increasing the AHF temperature had small effects on the penetration distance, and the SMD obviously decreased with increasing fuel temperature. The spray cone angle increased slightly, and the spray area decreased. The results showed that low-volatility AHF for safe and stable engine combustion could be achieved with air-assisted LPDI. In addition, the efficient atomization of AHF can be effectively implemented through the combined adjustment of injection control and physical and chemical parameters.