Abstract: Macroscopic observation, chemical composition analysis, mechanical property measurement, metallographic examination and energy spectrum analysis were used to analyze the causes of 12Cr1MoVG steel nozzle cracking of desuperheater in heat recovery steam generator. The results show that in the process of repeated start-up and shutdown of the desuperheater, the temperature difference between the water flowing into the nozzle and the pipe body was large, which made the nozzle bear large alternating thermal stress and formed thermal fatigue cracks on the inner and outer pipe walls, leading to the cracking of the nozzle finally.