Two wave coupling experiments of doped KNSBN single crystals have been performed. The gain coefficient of two wave coupling Γ as a function of the crossing angle of the beam 2θ, and Γ as a function of the intensity of light / in various samples were measured. The effective photo-refractive charge density, the effective electro-optic coefficient and the hole-electron competition factor have been determined by model fitting. The analytical expression of the intensity-dependent gain coefficient is deduced. We use this expression to analyse the experiment data of Γ-I. Our result shows that high values of Γ(>15 cm-1) exist in some kinds of doped KNSBN crystals.