Abstract: X groove butt welding tests of 16 mm thick Q345R steel plate were carried out by using submerged arc welding, and the effect of heat input on the microstructure and mechanical properties of the welded joint was studied. The hardness of cross section of the welded joint was measured and the hardness distribution under different welding heat input was obtained. The yield strength and tensile strength distribution of the joint were calculated by using the empirical formula between hardness and strength. The results show that with the increase of welding heat input, the penetration and reinforcement increase, the volume fraction of primary eutectoid ferrite in superheated zone of the weld increases gradually.The hardness of the weld and heat affected zone is higher than that of base metal, and the maximum hardness of the joint decreases with the increase of the heat input. The distribution of hardness and strength of weld and HAZ of the first pass is relatively uniform, and the gradient of hardness and strength between weld and HAZ of the second pass is large. When the heat input is the maximum of 41.0 kJ/cm, the yield strength and tensile strength of the welded joint are higher than those of base metal.