Abstract: Through the mechanical analysis of high backfill,the calculation formulae of the stratified backfill practice were deduced and reliability analysis of their stabilities were studied.In reliability analysis of high backfill,because the differential of status function was difficult to be obtained,a calculating method of reliability based on chaotic optimization was proposed,which was a new method of reliability analysis for complex status function in engineering practice.The mechanical researches showed that reducing length or increasing width of stope were propitious to improve stability of backfill and to lower filling cost.In order to evaluate the stability of backfill,a series of strength experiments of cemented tailings backfill both in the laboratory and under filling stope conditions were done,and their differences of strength were analyzed,as well as a conclusion was drawn that if the mechanical parameters in lab were used as the basis of filling design,the minimum safety factor should be 1.6 to 1.8,and the minimum index of reliability should be 1.8 to 2.0.Using the present method,the reliable indexes of backfill in stope No.3 in Anqing copper mine,Anhui province,were calculated,and the feasibility of filling design were evaluated.