In order to discuss the influence of different laser energy on the transformation mechanism of graphite into nano-diamond in a 1-D microscale heat conduction model, optimized graphite structure was simulated by molecular dynamics method based on density functional theory(DFT). The temperature distribution of graphite surface irradiated by laser was calculated by the finite difference method. Based on the sp3 bond that can make a distinction between diamond and graphite was discussed especially, the carbon atom bonding condition was studied according to the band gap of the density of states(DOS) obtained by energy coupling. The results show that a small number of sp3 hybrid carbon atoms can be formed only when the laser energy reaches 5 J, and with the increase of laser energy, the temperature of the irradiated graphite surface in the liquid phase increases, the free electrons in the carbon atoms can be easier to move to a bonding molecular orbital, and the electronegativity of the electrons will be enhanced, which boosts the sp3 bond polarity and helps to transform sp2 bond into sp3 bond. This study has important practical significance in improving the preparation efficiency of nano-diamond under laser irradiation in the liquid phase and exploring the preparation mechanism of nano-diamond.