Abstract: In magnetic particle testing, if the color contrast between magnetic powder and the workpiece surface is small, it is easy to cause some small defects being undetected. In order to improve the defect detection rate of magnetic powder, the contrast agent can be sprayed on the surface of the workpiece to improve the contrast. The effect of contrast enhancer thickness on defect detection rate was studied experimentally. The results show that the contrast enhancer with a certain thickness can improve the contrast but it has little effect on the defect detection rate.