Abstract: Following the thermal generator units being put into work, surface defect detection is needed for the axle bush of an air cylinder after it is examined and repaired in order to discover the tiny defects in running process and try to deal with them timely. This paper tends to optimize the penetrant testing technique reasonably according to the actual running state, through which the tiny defects will be detected effectively. The improved technique both satisfies the requirement of testing quality and enhances the flaw detecting ability as well.