Utilizing the approximate entropy algorithm, the changes of approximate entropy of electroencephalogram (EEG) θ rhythm (Apen θ) about 19 healthy volunteers before and after 5 Hz frequency photon stimulation (light repeatedly inducing 5 times per second) are analyzed. The results of T-test(P<0.05) show that the 5 Hz photon stimulation can significantly reduce the Apen θ of EEG θ rhythm. These changes can be obviously observed at the forehead, parietal, temporal and occipital region, and the most significant changes happen at temporal region and occipital region. The average Apen θ values about left and right brain EEG are both reduced after photon stimulation, and the Apen θ about right brain EEG is reduced more obviously. It can be concluded that Apen θ induced by external optical stimuli can describe different EEG rhythm characteristics and quantitatively show the extent of photon stimulation.