Abstract: Objective Certain new ultra-high strength steel is widely used in special equipment vehicles as a protective material, the aim is to predict the temperature change in the welding process and the residual stress after welding. Methods The numerical simulation of laser welding of 4.5 mm thick new ultra-high strength steel is carried out. The influence of different heat source models on the welding process is studied by means of temperature-stress sequential coupling. The appropriate heat source model is determined by comparing and analysising the simulation results and experimental results, and the temperature field and residual stress field of the welding process are simulated. Results The Gaussian body heat source model has high precision and can describe the welding process accurately. The heat affected zone of the new ultra-high strength steel by laser welding is very small, and the temperature gradient near the weld is large. Melting area of laser welding is small, the heating is rapid and the peak residence time is short. Residual stress at the weld is high, up to 304 MPa. Conclusion The Gaussian body heat source model is more accurate for the finite element analysis of laser welding. The welding numerical model has certain reference value for the prediction of weld residual stress and the microstructure change of weld.