In determining the dynamic thermal performance of a flat-plate solar collector, when the instantaneous solar irradiance changes sharply at one moment, most of the existing models cannot accurately predict the momentary thermal characteristics of outlet temperature and useful heat gain. In the present study, an analytical model in the form of series expansion is put forward to depict the momentary thermal characteristics of flat-plate solar collectors. The analytical model reveals that, instantaneous useful heat gain of a solar collector at one moment consists of the steady-state useful heat gain and corresponding thermal inertia correction. The model is then validated by the experimental data. It indicates that the analytical model can properly predict the dynamic thermal performance of the solar air collector. Besides, the model pertains to other types of solar thermal collectors, if they can be tested by the steady-state test method. (C) 2014 Elsevier Ltd. All rights reserved.