In order to obtain optimum thickness of VO2 thin films in laser protection，optical constants of VO2 thin films at low and high temperature were measured by ellipsometry on Si substrate，and optimum thickness of VO2 thin films in laser protection was calculated based on the transmission theory of thin films with absorptive properties.Then VO2 thin films with specific thickness were prepared by DC magnetron sputtering，and FITR was employed to test and analyze the infrared transmission properties.The results show that the infrared transmittivity of the VO2 thin films with specific thickness has an obvious phase transition property.The relative change of infrared transmittivity rate at 3~5 μm can reach 99%.The measured infrared transmittivity at 10.6 μm before and after phase transition are 67.2%，4.2% respectively，and the rate of change is 93.8%，which are approximate to the calculated data 66.4%，3.3% and 95%，and prove the accuracy of the calculation method.The VO2 thin films which are designed by the calculation method of optimum thickness can be ideal material for laser protection of infrared detector.