In real engineering of solar thermal applications, it needs considerable effort to determine the incidence angle modifier (IAM) of flat plate solar collectors, according to the test standards (BS EN ISO 9806, 2017; ASHRAE 93-2010, 2014). And the available method in the test standards is usually inapplicable to measure thermal performance of installed solar collectors with dust deposition effect in service. A quick measurement method is therefore presented to identify the IAM of flat plate solar collectors with less effort using a spectroradiometer. The quick method developed was validated with optical tests of a solar panel under the conditions of different incidence angles. It is inferred that the method not only helps to determine the IAM of flat plate solar collectors quickly without needing to run the collectors by energy power input, but also provides a pathway for assessing dust deposition effect on the thermal performance of installed flat plate solar collectors in service, as well as for determining the optical property attenuation of solar collectors in the long-term running.