Abstract: Recently, the nonlinear dynamics of the chemical reaction systems has been received great attention. This paper propose a model of chemical reactions, in which chemical tristablility can occur. The critical phenomena of tritability has been discussed. When the controlling parameter,u＞0, the systems has ristability, when u＜0, the systems has bistability only. And When u=0, the system has a critical point, at which instability is transformed into bistability comparing this critical point with the critical point of bistability, we find there are some differences between them, such as this point is unstable and the later is stable. But both can be also cast into the standard formulation of Landau （mean-field） theory of transitions.This reveals the featUre of second-order-like transition at the critical point of instability system and shows the generality of the different nonlinear chemical reaction systems, The Landau theory of transition, relaied imitially to the condensed matter in equilibrium state, is now also useful to systems, far from equilibrium.