L-beam is a widely used structural steel. The study of the three-stage bending deformation mechanism of L-beam under non-uniform cooling conditions is of great significance to ensure the quality, improve the yield of steel and reduce the stress level in the finished L-beam. However, it is difficult to investigate experimentally due to the high temperature and simultaneous changes of various influencing factors during the air-cooling process of the L-beam. Numerical simulation provides a convenient and feasible method to study the bending deformation mechanism of the L-beam. In order to investigate the effects of phase transformation and cooling shrinkage on the bending deformation of AH36 L-beam, a three-dimensional thermal-metallurgical-mechanical coupling model was established in this study, the microstructure distribution, hardness, strain, deformation status and stress during the air-cooling of L-beam were calculated. The results show that the microstructure distribution and deformation condition calculated by the model considering the phase transformation effect better agree with the actual situation. Considering the phase change effect is the key to improving the accuracy of the model calculation. The change of heat transfer coefficient caused by phase transformation affects the deformation tendency of the L-beam, while the transformation strain affects the amount of deformation of the L-beam. The alternating changes in the sum of the transformation strain and the cooling shrinkage strain result in a change of stress state which ultimately also results in a three-stage bending deformation of the L-beam.