Abstract: in order to utmost mine the seam under the slope at the west terminal of XierSurace Mine in Shenli ceafiel, a method combined with the nunerical smulation and limit equilibriun method was aplid to analyze the 3D failre mode and stablity variation aw of the end slope under th diferent mining working slope,i ner dumping tracing distance. Theresuts showed that the slope at the west high all .as affected by the doublesupport role ofthe working igh wall and imner dumping site . The deformation and stability would have obvious size eff ct. The reducion of the tracing distance would be an efcive access to conrol the slope deformationand improve the stability of the slope. Th slip surace of the slope would approximate an ll soid surace,the long axis and short aisin the horzonaldre cion wouldbe increased in linear with the racing distance increased. The stability coecient of the sope would be reduced in a negative index law with the tracing distance increased and when the tracing distance was not over 100 m,the stability could meet the safety requirement Nhan the racing ditancereached 20 m.the size efiect ofth sope stability would disappear basically.