Abstract: The effect of heat input on microstructures and properties of weld metal of D36 steel used in offshore platform was studied systematically by using SAW process. The results show that for the primary solidification microstructure in the weld metal,with the increase of heat input,the average width of the columnar grain increases,however its proportion decreases gradually. For the secondary solidification microstructures in the weld metal,with the increase of heat input,the amount of the primary ferrite（ PF） increases,meanwhile those of both the bainite ferrite（ BF） and acicular ferrite（ AF） decrease. What is more,the polygonal ferrite increases and the grain boundary ferrite increases at first and then decreases with the increase of heat input. The hardness of weld metal decreases along with the increase of heat input.