In this paper, the effect law of laser defocus amount on the macroscopic mech. properties, microstructure characteristics and thermal damage of skin tissue was studied. The change of defocus amount will directly affect the laser focusing radius and laser power d., then affect the laser fusion performance of skin tissue. In order to explore this pattern, 9 groups of different defocus values (-2, -1.5, -1, -0.5, 0, 1, 0.5, 1, 1.5, 2 mm) were selected as variables to perform laser welding on the incision of skin tissue samples. The mech. properties were measured and analyzed by digital display tension meter and optical microscope, the microstructure of the sample was analyzed by Masson staining and SEM (SEM). The collagen content around the incision and the degree of thermal damage of different samples were calculated by image J, Matlab and other software. On the basis of the above data, the incision fusion performance of samples was deeply analyzed. The results showed that the fusion performance of skin tissue changes with the defocus amount, when under pos. defocus amount, larger defocus amount resulted in superior macroscopic properties while much lower peak temperature and mech. properties, when under -1 mm defocus, samples indicated severe collagen distortion accompanied with rapid decrease of tensile strength. When defocus amount was -1 mm, the highest collagen content reached 59.8 % accompanied with superior homogenization of microstructure and the tensile strength acquired the highest 32.47 N/cm2 which was the best fusion performance and the conclusion had also been verified in rats.