Abstract: Cracking and leakage occurred during the operation of oil and gas pipeline at the top of a petrochemical unit.Through the means of macro observation, chemical composition analysis, mechanical properties analysis, metallographic analysis, fracture morphology observation and energy spectrum analysis, the cause of failure was analyzed combined with the field process parameters. The results show that the tee crack of the top oil and gas pipeline is a strain aging embrittlement crack originating from the machining streamline of the inner wall of the tee. The crack initiated at the machining streamline of the inner wall of the tee will expand brittlely under the action of machining stress and residual stress. After installation and use, under the action of temperature difference thermal stress and vibration load, the crack continues to fatigue and expands until the remaining section is not enough to withstand the pressure load in the pipe, and the tear leakage occurs.