The photorefractive surface waves with photorefractive nonlinearity of diffusion, drift, and photovoltaic effect is studied numerically for the first time. We find out that the essential cause of photorefractive surface waves is diffusion mechanism, but not drift and photovoltaic effects. The photovoltaic effect only has strong influence on the profile of photorefractive surface waves; the applied external electric field can also change the profile of the surface waves, and even destroy the mode upon a threshold, which provides a possible method to transform the modes of surface waves between high-frequency modes and low-frequency modes.