Referring to the principle of Landau′s theory on phase transitions, the varied transitions and critical phenomena in multi-stable optical systems are discussed. It is demonstrated that the higher the order of the multi-stability, the more varity of transitions, and shown also that the multi-stable system possesses different routes to complete mono-stability. A classification scheme of critical points corresponding to some transitions or bifurcation is made according to the stability criterion. This classification is useful to the study of critical phenomena, especially in non-equilibrium systems including the multi-stable systems.