In order to study the internal mechanism of unsaturated optical stopping effect with trace doped tin in As2S8 amorphous thin film waveguide,we use the double-prism coupled technology to investigate the thin film waveguide samples in detail.The chemical bond structures of the thin film materials are analyzed.With the help of the electron energy band gap theory and the hybridize orbital theory,the dynamical model of unsaturated optical stopping is proposed.The numerical analysis results agree with the experimental data very well,and the phenomenon of unsaturated optical stopping effect can be predicted by the deduction of this model.It reveals the nature of the experiments.