Abstract: In this paper, the influence of heat input on welding temperature field and microstructure of maraging steel C300 TIG (Tungsten Inter Gas Welding, TIG) welded joints were carried out preliminarily. The results indicate that, under these experimental conditions, as the heat input increased, the weld penetration and weld width both increased. The dark band zone (HAZ 2, Heat affected zone 2) widened and was further away from the weld center line, as well as that the difference between the top and bottom of dark band zones was also increased. The distance between 760 ℃ isotherm and the heat source basically satisfies the relationship of r=0.465E+0.987 (mm). The cellular grain size also increased with the increase of the heat input and basically satisfies the relationship λ₂=3.324×E^2/3 (μm), and with high applicability in the case of small heat input. Under best welding heat input, not only a good weld shape can be obtained but also a higher degree of agreement between the calculated value and the measured one.