A indirect detection method for energy flux density distribution of fresnel lens focal spot in concentration photovoltaic system was proposed, an experimental apparatus for detection was built. The specific process of extracting the spot energy flow density distribution from fresnel lens focal spot gray scale image was introduced in detail after the scaling factor k between the pixel gray values of the camera and the flux density was demarcated. The detection precision of this method was verified. The results indicat that, the relative error of peak value energy flow density detected by this method is less than 2.1%. This method has advantages of easy operation and high accuracy which can been widely applied in detecting for energy flux density distribution of fresnel lens focal spot in concentration photovoltaic system.