Abstract: The connection pipe of a trace water spray thermostat cracked in a thermal power plant. Based on the real-time change curve of the wall temperature of the front and rear-end connection pipe of the thermostat, the mechanism of the crack fracture was studied through macro-inspection, chemical composition analysis, mechanical performance analysis, metallographic analysis, SEM analysis and other methods of the crack site. The analysis results show that there is no misuse in the raw material of the connection pipe of the trace water spray reducer, and the performance meets the standard requirements. The main reason for the crack is that the main crack area is close to the trace water spray thermostat, it is easy to be subjected to the cold-heat alternating thermal stress and thermal fatigue, and Microcracks germinat in the site of the maximum stress concentration of open defects such as the inner wall straight. During long-term operation, thermal fatigue damage accumulates, and microcracks expand into macroscopic cracks, resulting in crack leakage.