A zoom optical system for thin film damage threshold test is designed to change the size of the optimal focusing spot and the distribution of energy density by zooming, which solves the phenomena of gas explosion and “internal sculpture” in the thin film damage threshold（LIDT）test caused by the focusing lens in the defocusing mode due to the high energy of the laser. Experiments show that the focusing optical system designed in this paper, the optimal focusing spot size can be adjusted by zoom. When the pitch changes from 18.152 mm to 6.851 mm, the effective focusing spot diameter changes from 100 μm to 700 μm. The beam morphology meets the Gaussian distribution. The laser energy distribution in line meets the use of the requirements. The reasonableness and validity of the focusing optical system are verified. The focusing optics can be adjusted by zoom. The system uses zoom to change the focal spot size and energy distribution. The best focusing spot acts on the surface of the film system. The formation of laser damage inside the component is prevented. The probability of misjudgment of the threshold damage of the film is reduced. The accuracy of the laser damage threshold test is improved.