In order to measure the laser induced damage threshold to film quickly, accurately, and quantitatively, the measurement system of flattop laser induced damage threshold to film was designed. The principle and method of damage threshold measurement were introduced, and the energy density search method based on the combination of binary search and sequence search was proposed. According to the light splitting ratio of spectroscope and indicating value of energy deteclor, the irradiation laser energy was solved, using the CCD imaging method, the laser facula area which irradiated on the film surface was measured precisely. Based on wavelet transformation, the film damage was identified accurately by image processing. The coordinates of energy density and damage probability was established, and fitted by least square method, the laser induced damage threshold was calibrated. Experiments were carried out on 45° reflection films which were irradiated by Gauss beam and flattop beam, the measurement results show that the damage threshold of Gauss beam irradiation was 9.95J/cm2 and the flattop beam irradiation was 13.98 J/cm2，the damage threshold irradiation by flattop beam was 40.5% higher than that by Gauss beam.