Experiment of automobile front crash indicates that the rail is the most important component to absorb impact energy. Not only the size and material can affect energy absorption of the rail,but also the quality and the distribution of the spot welds can have important influence. A research on impact of front rail by computer simulation is carried out in this paper. A Monte Carlo simulation method is used for generating the statistics distribution function of spot welding location in the front rail. The effects to the collapse mode and energy absorption are analyzed. The results show that corresponding to the variation of spot weld location,the difference in energy absorption can be more than 8%. The deformation mode of the front rail is also strongly affected.