The metal oxide semiconductor films have been used as sensing medium in the opto-chemical sensor based on surface plasmon resonance. The influence of the optical parameters of Ag and sensing films on the sensor sensitivity is analyzed. By optimizing the optical parameters of intermediate layers between the metal films and the sensing films, the sensitivity can be increased further. Experimentally, SnO2 and SiO2 films were selected as sensing and intermediate layers and the gas sensing tests to NH3、C2H5OH and C3H8 were made. The results indicate the sensor with optimized structure has a higher sensitivity.