According to the cracking of steam pressure sampling tube at the inlet of high pressure cylinder of an ultra-supercritical unit in a thermal power plant, the cracking mechanism of steam pressure sampling tube was studied by means of macroscopic inspection, metallographic test, hardness inspection, room temperature tensile test and SEM test. The results indicate that the inner wall microstructure of the cracked steam pressure sampling tube is stratified, and the microhardness of the core is higher than the standard requirement. There are many spot-like or strip-like endogenous inclusions in the sample core, so it can be determined that the principal reason for the cracking of steam pressure sampling pipe is the poor manufacturing quality of the pipe.