To meet the backfill strength requirements for the second step of retreat mining in the Peji Copper-Gold Mine area in Serbia and to reduce the economic cost of backfilling, it is necessary to optimize the backfill plan and the mix ratios of the backfill body for the mining area. Theoretical calculations have been used to preliminarily determine the strength of the backfill body in different parts of the second mining area, and several sets of indoor backfill body tests have been conducted. The tests indicate that as the mass concentration and the cement-to-sand ratio increase, the uniaxial compressive strength of the backfill body gradually increases. When the mass concentration is 68% and the cement-to-sand ratios are 1:6 and 1:8, the strength of the backfill body can meet the calculated requirements for backfill strength. Based on this, on-site industrial trials were carried out. The results of the on-site industrial trials show that using a backfill plan with a cement-to-sand ratio of 1:6 at the bottom of the mining area and 1:8 at the top and middle can meet the strength requirements for the second step of retreat backfilling. At the same time, the backfill cost is reduced by 2.53 yuan/t compared to the original backfill mix ratio. The research results can provide a reference for the backfill plans of similar mines.