In this study, on the basis of the assumption of “random-way” and “size-dependent-chosen” evolution, we reconsider an evolving food web model and discuss the mechanism of diversification, stabilization, and extinction of species. By adopting the size-dependent evolutionary system, we derived a particular evolutionary pattern of the diversity that maintained equilibrated state over the long term. And we confirmed that size-dependent evolutionary system is stable whereas size-independent evolutionary system raises the fluctuation and extinction of species diversity. The results implied that considering the size-dependent system would be essential in establishing food web numerical model.