Abstract: Ultrasonic surface wave can detect the surface crack of roll effectively，but the problem of how to determine the depth of the roll surface crack has been a major problem faced by test workers because surface crack is usually very shallow, meanwhile the current ultrasonic instrument can not precisely determine surface crack. This paper obtained the relation curves between the depth of crack equivalent d and echo db difference Δ in surface wave detection by calculation, which was applied to various types of Ma Steel roll in service testing, and achieved good results. Meanwhile through contrasting the d-Δ relation curves and practicing test, it proved that the detection sensitivity adjustment method in GB/T 23904—2009 standard should not be used for the surface wave detection in service work roll with minimum grinding of 0.2 mm each time. The d-Δ relation curve made the judgment of the roll surface crack depth more simple and accurate, and also it had some guiding significance for the surface wave detection of the work roll in service.