Abstract: Test surfaces are usually rough in ultrasonic flaw detection.If ordinary couplants are used,the coupling loss is high,the penetration ratio of sound energy is low and the detection efficiency is not good.This paper demonstrates a couplant suitable for ultrasonic flaw detection on rough surfaces.It's acoustic properties are superior to other kinds of couplants.It is very effective when used in ultr as onic flaw detection of nodular cast iron crankshaft.Due to the simple preparation methods,low cost and less loss,it can greatly improve ultrasonic flaw detection sensitivity and so has a very good prospect.