In order to explore the mechanism of fuel droplet impact on surface in diesel engine small-size combustor, a numerical model was developed using coupled level set and volume of fluid method including heat transfer and contact resistance. This model was verified by using experiments. The effect of wettabilities and the mechanism of droplet fluid and heat transfer as well as splashing during spreading were obtained according to results analysis. The theoretical condition of splashing is developed based on energy conservation equation. The results show that when the droplet contacts with the surface, it presents surface oscillation, spread and splash as the impact velocity increases. Spreading factor is closely related to impact velocity, the higher the impact velocity is, the larger the spreading factor is. The dimensionless time of maximum spreading factor is uncorrelated to impact velocity, the higher impact velocity is, the larger contraction is. The effect of wettabilities on spreading factor decreases with increasing impact velocity. The pressure gradient inside droplet is the main factor resulting in droplet spreading, breakup and splashing. Rayleigh-Plateau instability and capillary wave results in the shrink, breakup and splash from the liquid sheet. The conclusions are significant to spray control technology.