To further enhance the metal active gas （MAG） multi-layer welding quality of P355NL1 steel used in metro bogies， this paper conducts the MAG multi-layer welding tests of the butt joint of P355NL1 steel metro bogies， analyses the macro and micro morphologies of the butt joint structure， focuses on the study of the weld and the organisation of the heat-affected zone of each layer， establishes a MAG multi-layer welding model of the butt joint and performs calibration， and studies the distribution law of the temperature field of the MAG multi-layer welding. The results indicate that in the MAG multi-layer welding experiment， the heat input from the subsequent weld passes two phase transformations and recrystallization， which disrupts the columnar grain structure in the previous weld pass. This process leads to the refinement of the grain structure， forming fine ferrite grains and a small amount of pearlite. The double ellipsoid heat source model effectively simulates the welding process of P355NL1 steel in MAG multi-layer welding. The maximum temperature significantly increases during the welding of the upper layers. Therefore， controlling the welding temperature of the filler layers is crucial for grain refinement in multi-layer welding processes.