Abstract: A new kind of optically accessible constant volume combustion apparatus was successfully developed, which can be used to simulate the high pressure fuel injection characteristics and the spray/wall impingement mixing process under the operating environments of high temperature, high pressure and high density in the cylinder of a modern diesel engine at the time of fuel injection only by igniting the controllably pre-filled combustible mixture in the apparatus whose body need not be heated up to higher temperature. With the help of this apparatus, the diesel fuel injection spray characteristics and the mixing process of spray impingement can be measured by means of planar laser induced fluorescence (PLIF) or other optical methods. The experiments show that this apparatus can be used to simulate the in-cylinder high pressure and high temperature conditions up to 6.5 MPa and 1000 K. The theoretical computation method of each partial pressure of the pre-filled mixture components when simulating different pressures and temperatures and the pressure-time curves achieved by experimentation were given. This apparatus provided an essential prerequisite for measuring spray characteristics and spray/wall impingement mixing process of modern diesel engines.