Abstract: After about 10 months of service, a buried oil pipeline in a city cracked and leaked at the welded elbow. The causes of cracking of welded elbow were analyzed by macroscopic observation, chemical composition analysis, scanning electron microscope analysis and metallographic examination. The results show that the improper material selection of welded elbow led to the formation of hard and brittle martensite in the heat affected zone after welding. Martensite was sensitive to hydrogen embrittlement cracking. During the service process, the leakage of the drainage system on the front side of the pipeline caused the lower foundation soil to be dispersed, and there was no support under the elbow, resulting in tensile stress in this area. Under the action of tensile stress, hydrogen embrittlement delayed cracking occurred in the welding heat affected zone of the welded elbow.