Abstract: In order to meet the requirements for load reduction and backfilling at the top of the underwater immersed tube tunnel，the power plant cinder was used as the main component for the preparation and underwater backfilling of cinder-based foam lightweight soil.Based on the relevant physical and mechanical performance indicators as the characterization basis，the water immersion durability of cinder-based foam lightweight soil was systematically studied，so as to optimize the pore structure of cinder-based foam lightweight soil.Through standard curing and water immersion conditions，the mass water absorption，compressive strength，softening coefficient，etc.were checked regularly，and the variation of compressive strength and softening coefficient with immersion time was used to reflect durability of foamed lightweight soil.According to the test results of different curing ages，the main factors affecting the mass water absorption rate and softening coefficient of cinder-based foam lightweight soil were systematically studied，the relationship between mass water absorption，softening coefficient and density grade，cement content was analyzed.On this basis，the water immersion deterioration mechanism of cinder-based foam lightweight soil was analyzed.The relevant research results provide an important reference for the underwater backfilling of cinder-based foam lightweight soil.