When two coherent beams illuminate into the ethyl orange doped polymer film with small angle and equal intensity, the phenomena of self-diffraction occur. The dynamic characteristics of self-diffraction efficiency were explored experimentally. The results are the following: 1) The diffraction efficiency η increases monotonously from zero to a stable value ηs with the illumination time illuminated by lower intensity (I is 100 mW/cm2). 2) The diffraction efficiency η increases to a maximum ηm at first and then decreases to a stable value ηs illuminated by higher intensity. The phenomenon is analyzed theoretically by the model of population grating, which is formed by the molecules populations of trans and cis modulated by the coherence optical field. The theoretical results are consistent with the experimental results.