Diesel engine is basically based on diffusion combustion, so mixing process and mixing quality of the oil and gas in cylinder have emerged as the keys to improving fuel economy, power and emission performance of diesel engine. Mixture formation process in cylinder of diesel engine is a complex transient process, which is influenced by fuel injection system, gas flow and the geometry of combustion chamber. Spray characteristics of a high speed direct-injection diesel was investigated, using numerical and experimental methods, which include two groups of spray photos test based on a high speed camera test bench. In addition, transient flow, spray and combustion inside the injector of diesel was simulated by a CFD simulation software. And simulation model was verified through comparison of numerical and experimental cylinder pressure: the overall error is within 5%. Also, numerical investigation was performed under various injection pressure and initial pressure in cylinder and these numerical results are compared with experimental data to further validate the model, after which the influence of injection pressure and initial pressure in cylinder on spray characteristics was studied. Moreover, the effect of initial vortex ratio in cylinder, nozzle diameter and geometry of the combustion chamber on mixture formation was presented and discussed, respectively. Results show that in-cylinder parameters of the flow field, fuel injection system parameters and geometry of combustion chamber have certain effect on mixing process, mixing uniformity and speed of the oil and gas inside the cylinder, which could contribute to optimization of mixture formation process inside diesel.