In order to investigate the characteristics of internal diffusion of foam concrete， AC impedance spectroscopy of foam concrete was tested by electrochemical method at different water temperature， the presence or absence of foaming agent and different immersion time. AC impedance spectroscopy of foam concrete and ordinary concrete were researched. The results show that due to existence of inner bubbles in the foam concrete， its microstructure and characteristics of diffusion are different from those of ordinary concrete. In AC impedance spectroscopy， its Nyquist plot is of hyperbolic tangent type rather than Randles type as seen for the ordinary concrete. By correlation between the characteristics of diffusion and the hyperbolic tangent function， diffusion coefficient and depth of diffusion layer can be calculated.