Coal-based synthetic natural gas slag （CSNGS） was used as supplementary cementitious material， and the influence rule and mechanism of incorporating CSNGS on the rheological properties， mechanical properties and microstructure of cement composite cementitious materials were systematically investigated using rotational rheometer， flexural and compressive tests， X-ray diffraction （XRD）， scanning electron microscope-energy dispersive spectrometer （SEM-EDS）， Fourier transform infrared spectroscopy （FTIR）， and thermogravimetric analysis （TG） . The results show that the rheological properties of composite cementitious materials can be improved by mixing appropriate amount of CSNGS， and the yield stress and plastic viscosity of the slurry can be reduced by 27.2% and 35.0% respectively when the CSNGS content is 10.0%. The incorporation of CSNGS helps to improve the flexural strength of the composite cementitious material mortar， The 28 d flexural strength was 8.9 MPa when the CSNGS content is 20.0%， representing an increase of 13.8% compared with that of the cement mortar. In addition， the 28 d compressive strength of composite cementitious material mortar can reach more than 88% of cement mortar when CSNGS is mixed in 0%-30.0%. The results also show that with the incorporation of CSNGS， the gels and Ca（OH）₂ content decrease， n（Ca）/n（Al+Si） ， n（Ca）/n（Si） ， and n（Si）/n（Al） in the gels decreases， the content of ettringite （AFt） increases， and calcium silicaluminate hydrates （C-A-S-H） are generated in the reaction products.