In this paper,in view of the weakness of staticness and locality in the rational agents'theory of Rosenschein and the lack of description of single agent in the computational ecology of Huberman,the authors introduce the time parameter, on the basis of the rationality assumptions of the system, to give a formal description of the interaction between the rational agents and the system in a class of open DAI system,using the probability and the differential equation,so as to research the properties of dynamical behavior of open systems.