Abstract: The study seeks to obtain the optimal strength requirement for the backfill body in downward small sublevel stope,and optimize the optimal strength for the backfill body in the process of mining while guaranteeing safe stoping conditions,so as to lower filing cost and realize the best economic benefits.The paper analyzes and calculates the strength requirements based on the mechanical parameters of the ore rock-backfill body in Karatunk Copper-Nickel Mine.Based on the suggested stope structure parameters(length,width,and height are 30 m,8 m,and 12 m respectively),and the structural designs of the bottom layer,roof connection layer,and uncemented backfill body,the study uses Flac3D software to construct a 3D numerical model of ore rocks.The stability of backfill bodies with different strengths is studied when the roof is revealed downwards,to determine the numerical value of the minimum roof strength requirement of the backfill body in the process of downward mining;the difference and its causes of the backfill body strength calculation and numerical simulation results are comprehensively compared and analyzed,and the practical field situations are considered as well,before the downward backfill body strength and its structure design scheme are proposed,which realizes the coordination of stope structure parameters and backfill body parameters.