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A large amount of wet-discharged fly ash has caused serious harm to the ecological environment, so the utilization of fly ash has received attention. This paper analyzes the formation of products of fly ash–lime system under the autoclave process by X-ray diffraction (XRD) analysis and thermogravimetric–differential thermal (TG-DSC) analysis. The hydrothermal reaction product generation was quantitatively analyzed using the hydrochloric acid selective dissolution method to quantify the degree of reaction of fly ash in the pressure evaporation specimens in combination with the reaction degree of lime in the autoclave specimens. The hydrothermal reaction products were calcined and hydrated, and the mineral composition of the calcined products and the mechanical and microstructure of the hydrated products were analyzed. The results show that hydrothermal reactions occur in the fly ash–lime pressure evaporation system to produce C2SH, C3AH6, C3ASH4, and other products. The optimum ratio of lime is 22%, and the appropriate autoclave parameter is 140 °C for 8 h. Under this condition, the reaction degree of fly ash is 15.39%, the reaction degree of CaO is 78.63%, and its f-CaO value is 4.93%. The formation of C2SH in the hydrothermal reaction ranged from 14.33% to 18.53%, and the formation of C3ASH4 ranged from 14.06% to 15.26%. The hydrothermal reaction products were calcined at 850 °C for 1.5 h to produce new gelling materials with gelling phases, such as C12A7, α’L-C2S, β-C2S, and C2AS. The compressive and flexural strengths of the new gelling materials reached 34.4 MPa and 6.4 MPa, respectively, at the age of 180 days.