In this paper, the finite element method was used to study the thermal cycle, the evolution law of thermal stress-strain and the mechanical mechanism of the fiber laser welding and CO₂ laser welded superalloy Inconel 617. The results show that the accuracy of the heat source model for laser welded superalloy Inconel 617 with the rotating body heat source, the rotational body heat source rotational logarithmic body heat source and 3D gauss heat source is good. The cooling rate at the neck shrinkage of the HAZ is the lowest. When heating above 1280 ℃ in the HAZ, constitutional liquation was found inside the grain and at grain boundaries. There was no significant decrease in Von Miss stress of fiber laser welding and CO₂ laser welding for superalloy Inconel 617. The first principal strain of HAZ for fiber laser welding is increased gradually, while the first principal strain near the HAZ neck shrinkage in CO₂ laser welding is increased significantly. With the decrease of laser welding heat input, the sensitive strain rate of HAZ liquation cracking gradually increased. Compared with the Von-mises stress and the first principal strain of laser welding in HAZ, the liquation cracking sensitive strain rate is more suitable to explain the main factors for the formation of liquation cracking in laser welding of superalloy Inconel 617.