In order to explore the feasibility of the application of tailings powder in cement-based materials and improve the utilization rate of lithium tailings resources, the performance of tailings powder-fly ash-cement composite cementitious system was studied by using tailings powder produced in the process of lithium extraction and fly ash to partially replace cement. The results show that the flow performance of composite cementitious system can be improved by adding an appropriate amount of lithium tailings powder and fly ash. The early compressive strength of composite system is slightly reduced by adding lithium tailings powder and an appropriate amount of fly ash to partially replace cement, but it later strength is significantly increased. Compared with the control group, the 90 d compressive strength of experimental group with 5% (mass fraction) lithium tailings powder and 10% (mass fraction) fly ash increases by 9.75%, reaching 55.15 MPa. In the later stage of hydration, the composite activity effect and filling effect can be effectively exerted by mixing lithium tailings powder and fly ash, which can promote the formation of hydration products and fill them into macropores, increase the proportion of capillary micropores (< 10 nm) in cement paste, so as to refine the pore structure of cement paste and improve the density of system. Macroscopically, the compressive strength increases.