Local echo equivalent is found out of limits in an ultrasonic flaw detection of 60 kg/m U75 V rail welded joints in a rail welding plant,while the drop hammer test results of the joint are qualified. To explore the causes,the fractures after repeated drop hammer tests are analyzed by optical and electron microscopy and energy disperse spectrometer. The results show that: grey spots on the fracture surface are the main reason for excessive local echo equivalent of ultrasonic wave,and the two grey spots are caused by slag residue in rails and silicate inclusions not being extruded during welding respectively. It is suggested that the welding parameters setting should be further optimized