A study was conducted on the pore structure characteristics of foam concrete by mercury intrusion porosimetry(MIP), X ray computed tomography (X CT) and vacuum saturated approach. The 2D slice images obtained by X CT were analysized by Image Pro Plus software; and the porosity, pore size distribution and pore shape factor of foam concrete was characterized by image analysis software. The effects of dry density level and fly ash content on pore structure, compressive strength, water absorption were studied. The test results show that the average pore size and shape factors increase with decreasing dry density levels; an appropriate content of fly ash can improve the pore structure and mechanical properties of foam concrete.