The optical anisotropy caused by photo-induced isomerization of azo polymer has been investigated by experimental measurement. And a method to influence the photo-induced isomerization of azo polymer has also been reported. Silver nanoparticles with different spacing distances have been doped into the azo polymer, and the He-Cd laser with wavelength 442 nm has been used to excite the plasmon resonance effect of the silver nanoparticles. The intensity of electromagnetic field around the nano-particles has been enhanced while the mass concentration of Ag is 0.12 μg/ml and the plasmon resonance of silver nanoparticles in the thin sample is triggered. The conversion efficiency of photo-induced isomerization is also advanced greatly. In addition, we changed the mesogenic side groups of azo polymer and investigated the influence of interaction between mesogenic side groups and silver nanoparticles on the photo-induced isomerization. And a method to efficiently control the photo-induced isomerization effect has been presented.