Circulating fluidized bed fly ash (CFB fly ash) can form a high-strength solidified body by adding water to harden itself and has the potential to be used as roadbed. However, the hydration of f-CaO and II-CaSO4 in CFB fly ash to form ettringite and dihydrate gypsum causes the volume expansion of the solidified body, which may affect the volume stability of the subgrade. Therefore, the effects of standard curing time, sulfur and calcium content of CFB fly ash and compactness on California bearing ratio (CBR), water absorption and CBR expansion rate of CFB fly ash solidified body were studied in this paper. And the expansion mechanism of CFB fly ash in dry and hard system was studied. The results show that the expansion of CFB fly ash solidified body mainly comes from the water absorption expansion and hydration expansion of CFB fly ash. The longer the standard curing time is, the larger the CBR value of the consolidated body and the lower the water absorption and expansion rate are. The CBR value and water absorption rate tended to be stable and the expansion rate did not change at 7 d of standard curing. The higher the sulfur and calcium content of CFB fly ash is, the larger the CBR value of the consolidated body, the longer the expansion stability time, the higher the expansion rate after stabilization, and the lower the water absorption rate are. With the increase of compactness, the density of solidified body increases and porosity decreases. Therefore, the CBR increases gradually and the water absorption gradually decreases. Due to the increase of the constraint effect of CBR on the expansion, the expansion rate of the solidified body decreases gradually.