Abstract: Cracks are easy to be produced in the heat-affected zone of the fillet weld. If the cracks are not deep, they would be misjudged and neglected according to the magnetic particle testing. To ensure the safety operation of the equipments and to find this kind of defects promptly, exact detection technology and method would be taken. This article summarizes the reasons of forming the cracks and the importance of the inspections. This article especially provides the technology and method of the magnetic particle inspection. And a plenty of inspections and verifications are made to get satisfied effect.