Abstract: This paper mainly studies how to use the ultrasonic flaw detector for the determination of the thickness of composite plate of stainless steel. Composite plate of stainless steel is often used in structures, and the plate is composed of two different materials, so ultrasonic thickness gauge can not measure its thickness. By analysis of manufacturing process of the composite plate, the author found that there might be some small amounts of impurities (such as air, dust and other impurities) in between the interface of the two materials. In order to confirm that, the author carried out the ultrasonic flaw detector test and found that ultrasonic reflected wave exists at between the basis and complex layer of the composite stainless steel plate ( 16MnR+314L ), proving the correctness of the above-mentioned conclusions. Finally the author uses the principle related with the propagation velocity of ultrasonic wave for different materials to draw thickness correction formula in the measurement of composite plate, and makes an ultimate measure of the thickness of stainless steel composite plate.