As the mining operation of Chenmen Mountain copper mine gradually extents to a deeper stage, the stability of open pit slopes should be revaluated and new reasonable gradient is to be designed. Because of the big amount of calculation workload that should be applied to all the slopes, this paper proposed a method which is based on Newton Interpolation theory to reduce some numerous and repetitive job. Base on differential notes calculated by FLAC3D, a function precisely expressing the relation between the safety factor and slope angle was established, through which the safety factor of any slope of different gradients could be obtained directly. By comparative study of traditional methods such as the finite difference technique, it is indicated that the results from the new method that proposed in this study have a good agreement with results calculated by program FLAC3D.