Abstract: Q345 steel oil pipe of a power plant cracked and leaked during operation. By means of macro analysis, chemical composition analysis, hardness test and metallographic examination, the causes of pipe cracking were analyzed. The results show that due to the improper selection of elbow material, the material of straight pipe and elbow could not be correctly identified before welding, resulted in the improper welding process. The improper welding process resulted in cold cracks in the heat affected zone of welding, which eventually led to the cracking of Q345 steel oil pipe.