The study involves the preparation of foam concrete using fly ash modified by silane coupling agents, and investigates the effects of the amount of modified fly ash added on the waterproof and thermal insulation performance of the foam concrete. The results showed that when the mass fraction of silane coupling agent modified fly ash was 1.5%, the dry density of foam concrete decreased by 5.85%, the porosity increased by 3.67%, the water absorption decreased by 23.62%, and the thermal conductivity reached the lowest, 0.029 W/(m·K). Therefore, the proper addition of modified fly ash can effectively improve the thermal insulation performance and waterproof performance of foam concrete, but lead to a small loss of the mechanical properties of foam concrete. The 1.5% addition mass fraction of silane coupling agent can improve the thermal insulation performance and try to maintain the compressive strength of foam concrete.