The slopes of mountain spoil field located at the Zhengjiaya tunnel of the Yiba highway are taken as an example. The improved orthogonal analysis method based on the grey theory is introduced and then the numerical simulation software of Flac3d is used to analyze the sensitive factors which affect the stability of the mountain spoil field slopes. The result shows that strength parameters of the soil and the free surface shape are the highly significant influence factors which affect the stability of the mountain spoil field slopes in these areas. And the unit weight and slope rate have less influence on the stability of mountain spoil field slopes. The result is consistent with that of the gray method and the principal component analysis method. Therefore the results obtained from the improved orthogonal method are considered reliable.