Circadian rhythm makes physiology and behaviors of organisms adapt to the day and night cycle in the environment. Recent experiment indicated a PER-TIM interval timer in Drosophila melanogaster cytoplasm. In this paper, we present a possible mathematical model for this interval timer. The characters of the model are the formation of big clusters, fast exchange and different functions of phosphorylation of different sites. The experimental results on features of the PER-TIM interval timer of mild type and perL mutant and on the existence of three different types of temporal profiles of nuclear accumulation of PER and TIM in single cells are reproduced by our simulations.