The injection and mixture process of natural gas and diesel have important influence on combustion and emissions in dual-fuel natural gas engine. In this paper, the high pressure dual-fuel injection of natural gas and diesel was simulated by AVL-FIRE software and the spray model constants and the turbulence model were validated. In-cylinder high pressure direct injection and mixture process of natural gas and diesel were investigated by analyzing the effects of the following three factors on the mixture of natural gas and diesel including: different orifice distance, co-planar inclination angle and over-lapping angle. The results show that the longer parallel distance between natural gas orifice and diesel orifice means lower interaction and longer penetration which leads to better atomization of diesel. Co-planar inclination angle has almost the same effects with the orifice distance. Over-lapping angle is the most important factor among the three factors owing to the strong shear force between natural gas jet and diesel spray and optimal over-lapping angle can promote the atomization of diesel significantly.