Abstract: A certain medium pressure steam pipeline experienced cracking during use. The causes of pipeline cracking were studied using macroscopic observation, penetration testing, chemical composition analysis, mechanical property testing, metallographic examination, scanning electron microscopy and energy spectrum analysis. The results show that the cracking location of the pipeline was concentrated at the bottom of the pipeline, and unique cracks were generated on the heated surface. The crack branches were few, mainly transgranular cracking, and there were many high-temperature oxides present at the crack gaps. During the service of the pipeline, uneven ventilation and incomplete mixing in the mixing area caused cyclic thermal stress in the material, ultimately led to thermal fatigue cracking of the medium pressure steam pipeline.