Ultrasonic initial wave phrase reverse method was proposed to detect the depth of concrete crack.In this new method,diffracted character of low frequency ultrasonic pulse in concrete was used.The depth of crack was detect by simply moving the transducer to find the critical point of initial wave phrase reverse.Comparing with other testing method,the ultrasonic initial wave phrase reverse method is simple,audio visual,convenient,quick and without calculating by formula.The method also makes it possible to detect the crack depth in closely reinforced concrete by ultrasonic.