Influence of heat input on the penetration, microstructure and nano mechanical properties of laser welding joint is studied. The penetration and microstructure of joints are tested by stereo microscope and scanning electron microscope. The nanohardness and elastic modulus of joints are tested by nanoindentation. The result shows that the penetration increases with the increase of heat input, there is a linear relationship between penetration and heat input. Microstructure of weld seam is consist of full martensite when the heat input is below 94 J/mm2. Bainite and ferrite appear in the weld seam when the heat input is over 125 J/mm2, the volume of bainite and ferrite increases with the increase of heat input. With the increase of the welding heat input the nanohardness decreases gradually, the reason for the decrease is the formation of bainite and ferrite. The influence of welding heat input on the elastic modulus is not obvious, the modulus of elasticity is around 200 GPa.