Abstract: The influence of the welding heat affected zone（HAZ） of the conventional welding method and multi-layer welding on the drop weight test was investigated. By optical microscope（OM）,scanning electron microscopy（SEM）,the microstructures and fracture of the drop weight test sample were observed,the role of the HAZ in the drop weight test sample were analyzed. The results show that the drop weight test results are closely related to the dimension of the heat affected zone. With the increase of HAZ size,the nil-ductility transition temperature（NDTT） increases. The microstructures of HAZ mainly consisted of martensite and M-A component. When multi-layer welding is used,the content of martensite in fine grain HAZ increases,and the toughness of HAZ decreases remarkably. At the same time,multi-layer welding has no significant effect on reducing the HAZ size,which leads to the increase of NDTT of the drop weight test.