Abstract: The sampling pipe on main steam pipeline in a thermal power plant cracked many times. The cracking cause of the sampling pipes were analyzed by layout analysis of pipes and support hangers, and the calculation of pipe stress. The results show that the layout of sampling pipe and support hanger of main steam pipeline could not effectively support the thermal displacement of main steam pipeline and the pipe itself, resulting in the secondary stress seriously exceeding the standard, and the highest stress in the weld area of nozzle seat. When the local stress exceeded the fracture strength of material, cracks would produce and eventually led to the cracking and leakage of the sampling pipe.