In this study, in order to form a wide range of ambient gas density, Nitrogen and sulfur hexafluoride (SF6) are employed as ambient gas. Based on the common rail and high speed imaging system, the spray characteristics in extremely high ambient density and different ambient temperature are investigated in a constant volume vessel, and the influences of gas physical property on the spray characteristics have also been analyzed.The investigation results show that: the spray tip penetration and spray cone angle vary phenomenally with the increase of the ambient gas density. Due to the generation of shock wave, the results of the spray characteristics from two ambient gas densities are different nevertheless the same densities. Furthermore, the spray tip penetrations from the different ambient gases do not vary obviously from each other until the detachment of the shock wave from the spray tip. Hence, it is believed that the properties of the ambient gas between the shock wave front and the spray tip are influenced due to the detachment of the shock wave from the spray tip with energy and turbulence, thereby inducing the variation of the spray characteristics. Because of the existence of heat exchange between the spray and ambient gas atmosphere during the spray development process, more heat can be transferred to the fuel when the ambient gas temperature increases, thereby resulting the decease of fuel viscosity and further weaken the flow resistance suffered by the spray. Consequently, the spray tip penetration increases with the increase of the ambient gas temperature. A new equation has been proposed based on the empirical correlation proposed by Hiroyasu, which has a good behavior for predicting the spray tip penetration at a wide range of ambient gas density.