Abstract: P355NL1 steel has good plasticity, toughness and weldability, and is an ideal material for subway bogies. For medium and thick plate structural parts such as bogies, MAG multi-layer welding is generally used for connection. This welding method has the characteristics of large heat input, high energy utilization, low cost and few pore defects. However, the change of heat input affects the change of grain and temperature field of multi-layer welding organization, and because the complex thermal cycle process is difficult to observe by experimental means, simulation is an important means to explore its thermal process. In this paper, welding tests and simulation studies are carried out on MAG multi-layer welding of the typical T-shaped structure of P355NL1 steel rail vehicle bogie. Analysis shows that the rear layer weld has the effect of refining the grains of the heat affected zone of the front layer. The bottom layer weld has undergone multiple thermal cycles, and the welding of the upper layer weld has a heat treatment effect on the heat input of the lower layer weld. At the same time, the influence of parameter changes on heat input and thermal process is studied, and it is found that with the increase of welding power and the decrease of welding speed, the weld width and depth increase.