Through sieving and crushing, fly ash slag with the particle size range of 0.6 mm to 1.18 mm and 0.3 mm to 0.6 mm were obtained, respectively, and fine aggregate in the corresponding particle size range was replaced by the same volume of fly ash slag. The influence of fly ash slag on the workability and strength of mortar was analyzed, and the optimal alternative particle size range of fly ash slag was explored. Combined with scanning electron microscopy (SEM), energy dispersive spectrometer (EDS) and other methods, the strength change mechanism of mortar specimens after the replacement of fine aggregate by fly ash slag was analyzed. Based on the results of the optimal replacement particle size range of the mortar, the strength and frost resistance of the mortar concrete specimens were verified. The results show that: after sieving and crushing to obtain fly ash slag in the particle size range of 0.3 mm to 0.6 mm to replace the fine aggregate in this interval, the strength of the mortar specimens is the same as that of the benchmark group; sieving to obtain the fly ash slag in the particle size range of 0.3 mm to 0.6 mm to replace this interval, the strength and frost resistance of the concrete specimens are the same as those of the benchmark group. Under the calcium hydroxide environment provided by cement, calcium silicate hydrate is formed on the surface of fly ash slag, thereby increasing the cement and fly ash slag interface bonding strength, strengthening the interface area between the cement and the fly ash slag. The uneven surface is interlocked with the cement slurry to ensure the strength of the specimen.