Based on an exact solution of nonlinear coupled wave differential equations for photorefractive media, a conservative quantity which plays an important role in the process of deriving the exact solution is discussed. By elementary transformantions, several approximate expressions for intensity of the phase conjugate wave under different special conditions are obtained. As compared with other approximation methods reported, this approximation method is no need of establishing and finding solutions of reduced differential equations, therefore it is characterized by general suitability and simplicity.