Taking the optical system as a prototype, this paper demonstrates that a quasi-thermodynamic model for dissipative systems, satisfying the principle of detailed balance, may be developed, which is parallel to the traditional thermodynamics. The present systems are described by the generalized dynamical equation of nonlinear optical cavities, introduced in previous work of Ou Fa et al. In view of the quasi-thermodynamic model, the critical phenomena and phase transitions in optical bistability are discussed systematically and generally. By taking the advantage of Ginzburg-Landau's model, the fluctuations and correlations near the (second kind of) critical point of optical bistability are studied also. So the Landau's theory of phase transitions in equilibrium thermodynamics is generalized to the bistable dissipative systems far from equilibrium.