Abstract: The test and calculation methods for the dynamic fracture toughness of 40CrMnMo square drill pipe joint produced by narrow gap pulsed metal arc welding were studied.Combined with the use of square drill pipe,pipe size and welding characteristics,the repeated impact 3-point bending test was used to test the fracture toughness of the square drill pipe joint.The crack depth was measured by using a microscope.and K_Id values were calculated with a FORTRAN program.The experimental results show that K_Id of the weld is 96% of that of the base material,but K_Id of the fusion line is only 89% of that of the base material.The analyses on the fracture surface and crosssectional microstructure indicate that the bulky Widmanstatten structure near the fusion line is the main reason for a lower K_Id.