The phosphogypsum and copper tailings sand are mixed to prepare a mixture, and cement is added for highway roadbed filling, which can realize the large-scale utilization of solid waste resources and the purpose of “waste treatment by waste”. The geotechnical performance test of the mixture was used to explore the influence of the mass ratio of phosphogypsum and copper tailings sand on the mixture’s geotechnical performance, combined with modern analysis and testing technology to study the hydration-hardening mechanism and micro-mechanism of the mixture. The results show that neither phosphogypsum nor copper tailings sand can be used alone as roadbed filling materials because of the higher liquid limit of phosphogypsum and the lower strength characteristics of copper tailings sand. When phosphogypsum and copper tailings sand are mixed with a mass ratio of 4∶6, the geotechnical performance of the mixture was better. The hardening of the mixture is caused by the hydration of the cement. Hydration products such as ettringite and C-S-H gel can fill the gap between the particles of the mixture and form a compact aggregate, which causes the mixture to harden.