Abstract: The fly ash entrained concrete were designed and tested in 3. 5% NaCl solution under rapid freezing and thawing cycles，and then the concrete damaged layer thickness and relative dynamic modulus of elasticity were studied by the ultrasonic testing technology，and so on the static elastic modulus． The results showed that the deterioration speed of concrete performance was faster than later on the early period of salt-frost cycles． With the freeze-thaw cycles ongoing，the deterioration speed was reduced． All the static elasticity modulus，the relative dynamic elastic modulus and the salt-frost damaged layer thinkness have the same trend，which can characterize the modulus and the concrete salt-frost damaged layer thickness．