An ex-situ determining method was proposed for the optical constants of aluminum thin films precisely.Compared to the traditional in-situ method,the method had no need of the complex optical measurement apparatus attached to vacuum chamber and could do measurement simply and fastly.By using multi-wavelength fitting method,the ellipsometric and photometric spectra were fittd with a combination of optimal genetic algorism and a simplex algorism and the oxide layer formed on the aluminum in air was considered in the fitting process.With the combination of parallel genetic algorithm and least square method,the fitting speed and quality were obviously improved and the optical constants from the UV to visible range were obtained within an error no more than 2%.With these data known,an UV induced transmission filter with one cavity was designed.The tested transmission spectra are well consistent with that design in a peak transmittance error less than 1% at 265 nm.