Diesel-fuelled homogenous charge compression ignition (HCCI) engines are a new area of focused research as improvements are sought in internal combustion engine technology to reduce energy consumption and air pollution. This paper proposes a new method of utilizing the flash boiling injection effect of high-temperature diesel fuel to improve the fuel-spray evaporation and mixing in HCCI diesel engines. A visual test bench of quasi-stable state fuel injection spray was established, and the influence of high temperature flash boiling injection of diesel fuel on Sauter Mean Diameter (SMD) was tested by laser droplet size analysis. The SMD distributions of diesel engine fuel was measured under different injection pressure and fuel temperatures, and the flash-boiling spray effects on decreasing the fuel droplet diameter, and improving the spray mixture were verified.A Model ZS195, single-cylinder, direct-injection diesel engine was used to test the HCCI combustion efficiency. A section of high-pressure fuel supply pipe with heating and temperature controls was installed following the retrofitted HCCI fuel injection pump. The combustion and exhaust emissions of the HCCI diesel engine were tested for different fuel temperatures, and the effect of high-temperature flash boiling was clearly demonstrated by the test results. In addition, the diesel fuel temperature range to optimize fuel economy and exhaust pollution was identified. Tests were carried out to compare different fuel supply advance angles and injection pressures to ascertain the influence of engine structure and operating parameters on the combustion and emission characteristics. As revealed in the test results, the HCCI combustion diesel engine performance, including fuel consumption and exhaust emissions, were improved by high-temperature flash boiling fuel spray, by increasing the fuel supply advance angle to 32 deg crank angle (CA) before top dead center (BTDC), and increasing the fuel temperature to 160 - 200 deg C.