Abstract: After 14 months of operation, multiple cracks appeared near the weld of inner and outer walls of a boiler feed water preheater of an oilfield methanol plant. Through macroscopic inspection, chemical composition analysis, mechanical property test, metallographic examination, energy spectrum analysis and other methods, the causes of cracking of the boiler feed water preheater were analyzed. The results show that the contents of chromium and nickel in the material of the preheater were too low, which led to the increase of hardness and the decrease of toughness of the material. Meanwhile, the phosphorus content exceeded the standard requirement, which further reduced the plasticity and toughness of the material. The chromium-poor region of the grain boundary was highly susceptible to corrosion and microcracks produced. Under the action of the residual stress existing in the boiler feed water preheater material and the stress caused by the working internal pressure, the grain boundary microcracks continuously expanded and connected, and finally caused the preheater to crack.