The thermal deviation and steam temperature deviation are the main problems during the operation of superheater and reheater systems of large utility boilers, and also the main cause for the tube failure of superheaters and reheaters. The T-junction inlet and outlet mode of steam in the superheater and reheater system can result in obvious steam flow deviation and thermal deviation, and even the tube failure. An experimental study on the static pressure distribution in the T-junction was conducted to analyze the cause for the tube failure in the T-junction of the roof superheater distribution header in a 660 MW super critical boiler, according to the structure characteristics of the T-junction inlet of the distribution header and the arrangement of tubers of the roof superheater. The coverage of the vortexes and the static pressure distribution in the T-junction at different conditions were revealed. The main cause for the tube failure of the roof superheater is the vortexes in the T-junction. The effective countermeasure is put forward.