The problem of resource waste and environmental pollution caused by circulating fluidized bed fly ash (CFBFA) is becoming increasingly acute, and its resource utilization is imminent. In this paper, carbide slag (CS) and desulfurized gypsum (DG) were used to synergistically activate CFBFA and granulated blast furnace slag (GBFS) to prepare low-carbon cementitious materials and apply them to pavement concrete. The results indicate that with the increase of CFBFA content, the mechanical strength of CFBFA-GBFS composite cementitious materials at 3 and 7 d decreases gradually. At the age of 28 d, the mechanical strength of CFBFA-GBFS composite cementitious materials initially increases and subsequently decreases. When the CFBFA content is 40% (mass fraction, the same below), the 28 d flexural strength and compressive strength of CFBFA-GBFS composite cementitious materials are the highest, which are 7.4 and 32.8 MPa, respectively. When the CFBFA content is increased from 30% to 70%, the initial setting time of CFBFA-GBFS composite cementitious material is extended from 447 min to 753 min, and the final setting time is extended from 549 min to 802 min. The 28 d drying shrinkage of the specimen is the largest when the CFBFA content is 50%, which is 0.031%, respectively. The hydration products of CFBFA-GBFS composite cementitious materials mainly include hydrated calcium aluminosilicate (C-A-S-H) gel and ettringite (AFt). The interlaced growth of C-A-S-H gel and needle-like AFt in space refines the internal pores of the material, which is conducive to the improvement of strength. The 28 d flexural tensile strength and compressive strength of pavement concrete can reach 4.8 and 43.0 MPa, respectively, which meet the standard of intermediate load pavement concrete.