Vanadium dioxide thin films can be used in laser protection for it has the closest phase transition temperature to the room temperature. Before phase transition, its transmittance is high, and the detector can work. Phase transition occurs when the attack laser′s energy is absorbed, then transmittance decrease, and the detector can be protected. The film thickness has large influence on transmittance. Its influence can be analyzed by eigenmatrix method of absorbing thin film, calculating the transmittance before and after phase transition using refract index n and extinction index k. For the transmittance 75% before and 5% after phase transition by calculating the thickness, combining with the calculation of sputtering yield and sputtering rate, preparing time can be gotten. VO2 thin films were prepared on zinc selenide, the transmittance was measured by IR spectrophotometer, 79.2% before and 12.3% after phase transition. The thickness was measured by profile meter, and the result fits the calculation well. The calculation about thickness is doable.