An improved and generalized model was developed to predict the dissociation pressures of hydrate-forming gases and the complicated phase behavior of multicomponent hydrate systems. Based oil the van der Waals-Platteeuw theory with the Kihara spherical-core potential function, a new equation for the water fugacity in the filled hydrate phase is provided. Instead of vapor pressures of ice for conventional approaches, it adopts the molar enthalpy difference between ice and liquid water. Since it always maintains its unique form whether the hydrates coexist with ice or water, it can be successfully applied to predict the multiphase equilibria of simple and mixed hydrate systems. The fugacities of all components in vapor and liquid phases coexisting with hydrates were calculated by the Soave-Redlich-Kwong (SRK) equation of state incorporated with the modified Huron-Vidal second-order mixing rule. Lower and upper quadruple points and neighboring four three-phase curves around these quadruple points were described correctly by this model calculation. The Kihara potential parameters for the interaction between water and gas molecules are presented by fitting experimental dissociation pressures. Various types of systems involving hydrate mixtures containing either two guest components or inhibitors were extensively tested to examine the model's potential applicability to predicting the complicated phase equilibria.