Lithium slag (LS) powder and steel slag (SS) powder were used to partly replace the P·O 42.5 cement (PC) to prepare composite cement pastes. SEM, XRD and FT-IR were used to analyze the effects and mechanism of the two slags on samples. The results show that the replacement of part of cement with LS reduces the fluidity of paste, while the replacement of part of cement with SS is beneficial to improve the fluidity. LS promotes coagulation, while SS slows coagulation in paste. The fluidity and setting time of pastes are controlled by the combined effects of LS and SS. LS has more obvious advantages than SS in improving the mechanical properties of paste. When the water-binder ratio is 0.4, the compressive strength at 28 d of the sample mixed with 20% (mass fraction) LS reaches more than 62.3 MPa, which is about 23% higher than that of control sample. SEM results show that the microstructure of paste with 20% LS is denser than that of control sample at 28 d. XRD results show that the hydration products of the composite pastes are mainly C-S-H gel and calcium hydroxide. FT-IR results show that the peak position of Si—O bond has a red shift, and the H—O—H bond has a blue shift.