Abstract: In magnetic flux leakage testing (MFL), test result was affected by various factors. Surface roughness was the main factor influencing the sensitivity of detection. The aim of this paper is to analyze the effect of surface roughness on crack detection sensitivity by an experiment on surface roughness reference block with artificial groove injury in different depth, and to analyze detection results of different rough surface by comparing the SNR. The results showed that the SNR value decreased as the surface roughness increased, and a certain depth of the crack could not be clearly distinguished at a determined roughness, moreover, triangular serrated roughness elements were taken to simulate the convex portion and the concave portion in surface roughness model of solid, and analyzed distribution of magnetic field intensity from the perspective of the magnetic refraction, the simulation results gave a theoretical detection sensitivity of crack on rough surface.