A photo-crosslinked side-chain second-order nonlinear optical polymer was synthesized with bis-phenol-A as the polymer backbone, p-nitroaniline as the chromophore and cinnamyl group as the photo-sensitizer. The characterization of the polymer was made by nuclear magnetic resonance, infrared, ultraviolet-vis, and dy namic mechanical analysis methods. An electro-optic film was obtained by spin coating of the polymer and corona poling then photo-crosslinking reaction. An external electrooptic measurement system was established based on the film and reflective light path configuration, and successfully applied to measure the electrical signals propagating on the under-test circuits. The poled and crosslinked film showed high glass transition temperature (160 degrees C) and improved chromophore orientation stability. The measuring principle was analyzed by electro-optic tensor matrix and index ellipsoid methods. The results showed that the designed polymer film had the linear electro-optic effect, the voltage sensitivity of the system was measured to be 5 mv/root Hz at the condition of 1 kHz input signal.