Deterministic interface states in photonic crystals have attracted extensive attention from researchers in recent years due to their widespread applications in optical waveguides and other fields. This paper uses effective electromagnetic parameters to predict the existence of deterministic interface states in photonic crystals, which extends the Zak phase theory of band structures. The effective electromagnetic parameters are superior because they bring advantages of requiring no calculation of complex band structures and Zak phase and making the design of deterministic interface states in photonic crystals more convenient. Similar to surface plasmon polaritons, deterministic interface states can exist when the sum of the impedances of two photonic crystals forming the interface is zero. Therefore, the existence of deterministic interface states in anisotropic photonic crystals only depends on the effective dielectric constant and effective permeability in specific directions. In addition, it is found that deterministic interface states can exist and propagate only along specific interface directions by designing anisotropic photonic crystals with required effective electromagnetic parameters. The simulation results verify the accuracy of the effective medium theory for photonic crystals and the existence of interfacial states.