Abstract: Fractured happened to the joint weld of an downhole electric pump in a western oil field after using for less than one year. The fracture reasons of the electric pump joint weld were analyzed by using a series of physical and chemical performance testing methods and considering the actual field working conditions. The results show that the weld microstructure of the anti-back block and the pump body was austenite and δ-ferrite, and the microstructure of the welding heat affected zone was martensite with microhardness of as high as 500 HV0.5. The abnormal microstructure caused the weld embrittlement, and thus led to the low stress brittle fracture of the electric pump at the weld position. Finally, suggestions of optimizing welding process were put forward.