Based on the recent theoretical research on the open-circuit voltage of silicon photocell changing with the intensity of light，itpropose a theoretical model of a dynamic change of the built-in electric field．This model successfully explains the experimental phenomenon that the open-circuit voltage changes with the intensity of light in the test of characteristic parameters of silicon photocell．The validity of the theoretical model  was further testified by theoretical simulation and experiments．This provides a theoretical starting point for the classroom design and teaching content selection for the characteristic test experiment of silicon photocell．