Abstract：By using investigation information of slopes development，the grey system theory can be applied to forecast the unknown deformation of slopes. When season factors become the main influencing to the deformations of slopes，still using equal interval time series which will destroy complete season factor can not reflect the correct deformation orderliness of slopes and then the wrong forecasting will be made. But applying the time series divided by season factor to forecasting the deformation of slopes，the prediction results agree well with the real deformations，which is much more accurate than the former.
