The utility boiler external piping had been carried on the inspection and detection in the jurisdiction of the author.There are long cracks were founded in the upper and lower butt welds of the main steam pipeline with the Parallel steam valve.This paper mainly takes the inspection and detection of the main steam pipeline of 3 boilers and 2 turbines of a chemical self-owned power plant as an example,the causes of the cracks in butt welds of the front parallel valve of the pipe-main were analyzed,and some suggestions and preventive measures were put forward,in order to attract the attention of the users and inspectors,discover and eliminate the hidden dangers in time,and avoid the occurrence of the main steam pipe accidents.