A method to accurately evaluate optical thin-film damage threshold is presented. The poor coherences in time and space of amplified spontaneous emission (ASE) result in a very smooth beam profile in the near-field region and uniform intensity distribution of the focused beamlet in the far-field region. In order to increase the uniformity of the irradiation source and test the damage threshold with a greater precision, ASE beam is used to test the damage threshold. ASE is generated by a rod amplifier of the Ndglass in SG-II high power laser system. The pulse duration is 9 ns after an electro-optical switch with the output energy changing from a few millijoules to tens of joules. The spectral full width at half maximum (FWHM) is 1 nm. According to ISO-11254, the damage threshold of the TiO2 high reflection film using ASE is 15.1 J/cm2, which is higher than that of 7.4 J/cm2 tested by laser with pulse duration of 9 ns. So a more accurate evaluation of the samples damage thresholds can be obtained using ASE as the irradiation source.