Abstract: In the process of urban construction and underground development, a large amount of construction waste(CW) and engineering waste(EW) are generated. If a mixture of CW and EW can be used for backfilling the abandoned mine, it can not only effectively utilize CW, save land resources, but also promote the ecological restoration of the abandoned mine. The composition of CW is complex. In order to ensure the safety of the backfilling slope and to fully utilize CW, it is urgent to research the optimum ratio of CW and EW.Depending on the backfilling slope of the abandoned mine in Suzhou, laboratory research on the optimum ratio of CW and EW has been carried out, and then based on the results of laboratory research, GeoStudio is used to analyze the stability of the slope with different mixture proportions. The results show that the shear strength parameters of the mixture exhibit an exponential function relationship with the ratio of CW to EW.With the increase of the proportion of construction waste, the mixture's cohesion decreases continuously, while its internal friction angle increases continuously. When the ratio of CW to EW is 0.4, the slope under different conditions is more stable. Therefore, 0.4 is the optimum ratio of CW to EW for backfilling the abandoned mine. Under this ratio, the mixture has high strength and high clay content, which is conducive to the later greening of the mine pit.