During the construction of Beijing underground rail transit line, the cutterhead of the shield machine has to be repaired in the urban area. In the paper, the concept of dynamic fire under pressure is presented, the site conditions of operation under dynamic fire are described, the technical requirements for the safety, health and environment of the welders are analyzed, the welding method, welding equipment, welding materials and welding techniques under pressure are studied, thus a complete technological system is established. In the end, satisfactory effect has been achieved in the project.