We construct a microscopic model to describe the excited states of poly(2-methoxy, 5-(2'-ethylhexoxy)-p-(phenylenevinylene) in thin film. Within this model, we deduce that in the high energy region, the nature of excited states in the film is very similar to the species observed in solution phase. Moreover, we propose that the decay process of these excited states involves energy transfer, vibrational relaxation. and dissipation simultaneously, in contrast to the usual argument that assumes exciton migration occurs after vibrational motion reaches thermal equilibrium. As a result, the simulation of time-resolved photoluminescence spectra is in agreement with the experiment.