It is found in the inspection that bonding beryllium and copper together with two kinds of metals can lead to bad bonding, which seriously affects the safe operation of the equipment. In this paper, the residual stress distribution of the bonding block (beryllium side) at different depths is detected by using a probe of 15MHz, 10MHz, 7.5MHz, 5MHz, 4MHz. According to the propagation law of the critical refraction longitudinal wave and the ultrasonic detection system of the critical refraction longitudinal wave, the residual stress distribution of the bonding block (beryllium side) at different depths is detected. This shows that the critical refraction longitudinal wave with different frequencies can reflect the residual stress at different depths. By using this rule, the ultrasonic nondestructive evaluation of the stress gradient distribution along the depth of the surface of the bonding block can be made. The effect of stress gradient distribution on the bonding block is further investigated.