Q345E steel plates for express freight car bogie were welded by metal active gas arc welding（MAG） welding method. After post weld nondestructive testing,the fatigue performance tests of the qualified welded joints were carried out. According to the test results,the fatigue S-N curve of the welded joint was drawn up,the fatigue limit and standard deviation of the welded joint were determined by lifting method. The fatigue failure mechanism of the welded joints was determined by fracture surface analysis of fatigue specimens.The results show that the porosity and slag defects in the weld have significant impacts on the fatigue properties of the welded joints,which will become the initial positions of fatigue cracks and deteriorate the fatigue properties of the welded joints.