Abstract: Some area on the sealing surface of valve seat of suction valve on the east side of the four-stage cylinder of hydrogen compressor in a styrene plant fractured in operation. The fracture causes of the sealing surface of valve seat were analyzed by means of macro analysis, low power analysis, chemical composition analysis, metallographic examination, hardness test and scanning electron microscope analysis, and the corresponding measures were put forward. The results show that due to the improper heat treatment process, the fatigue crack was produced by the metal damage of the outer ring step on the upper surface of the valve seat and extended to both ends continuously in working process. Under the action of alternating load, the springs in valve broke, which resulted in more concentrated stress at the crack and decreased bearing capacity of the valve seat, accelerated the fatigue crack growth of the valve seat, and finally led to the fracture of some area on the sealing surface of valve seat.