Calcium carbide residue, as an industrial by-product with high Ca(OH)2 content, can accelerate the hydration process of alkali-activated composite cementitious material in cooperation with sodium carbonate. In this paper, fly ash and mineral powder were used as precursors of composite cementitious material, the effects of different ratios of calcium carbide residue (CCR) and Na2CO3 on the pore solution pH value and mechanical properties of composite cementitious material were investigated. In addition, the effect of co-activation of CCR-Na2CO3 on hydration process and microstructure of composite cementitious material was investigated by heat of hydration, X-ray diffraction, thermogravimetric analysis and scanning electron microscopy. The results show that with the increase of calcium carbide residue content, the pore solution pH value and mechanical properties of composite cementitious material increase first and then decrease. When the content of CCR and Na2CO3 are 6% and 9%(mass fraction), respectively, the pore solution pH value at 3 d and compressive strength at 28 d of alkali-activated composite cementitious material reach the maximum value, 12.95 and 26.8 MPa, respectively. The microstructure shows that alkali-activated composite cementitious material can generate more C-(A)-S-H gels under co-activation of CCR and Na2CO3, which makes the structure more dense.