Abstract: In order to improve the performance of X80 pipeline steel welded joints, the heat affected zone of the GMAW joints was studied by experiment and numerical simulation. A regression orthogonal experiment was carried out and the relationship between the weld size and process parameters was obtained. On this basis, the double ellipsoid heat source parameters were calibrated with the obtained weld dimension, and the temperature field was calculated. The thermal cycle parameters in heat affected zone under different heat input were discussed and finally the microstructure was observed. The results show that the size of the weld pool simulated by using the actual size of the weld as the heat source parameter is closer to the actual size, and the accuracy of the finite element model is also proved. With the increase of heat input, the peak temperature of the heat affected zone increases, the cooling time and residence time at high temperature also increase, but the increase is not obvious, and the grain size of the heat affected zone increases.