In order to analyze the microstructure evolution and grain growth processes of coarse grain heat affected zone(CGHAZ), fine grained heat affected zone (FGHAZ), and intercritical heat affected zone (ICHAZ), 12 mm SS400 medium plate was welded under different heat input. The varying rules of the HAZ width, microstructure and microhardness with the variation of heat input in welding HAZ and all sub areas were studied using the finite element method in combination with microscopic tests. A relationship between the heat input and HAZ width which follows the Boltzmann distribution was also presented. The result shows that when the steel was welded with heat input of 9.6~22 kJ/cm, the effect of the heat input on the width of HAZ and all sub areas was small; the width of HAZ increased when the heat input was higher than 22 kJ/cm, especially leading to the substantial increase of width in FGHAZ. Moreover, when the heat input increased, the hardness peak of the joint became higher in CGHAZ which was caused by a restraint of the lamellar bainite transformation and the widmanstatten formation, while the softened zone expanded gradually. Preheating can restrain the growth of the HAZ width for the welding with high heat input.