Abstract: By comparing the results of fluorescence penetration detection before and after cleaning of the small pitch bolts, it was found that the main reason why the cracks on the surface of the root of the bolts did not show fluorescence was that the dirt at the opening crack impeded the effective penetration.The detection rate of crack can be effectively improved by thoroughly clearing the bolt surface. Furthermore, crack detection ability can be improved by using high-sensitivity and super-high-sensitivity penetrant. Considering the accuracy and cost of detection, it is more suitable to use high-sensitivity penetrant for small-pitch bolt detection.