A novel LMR-based optical fiber sensor fabricated with SnO2 thin film is proposed in this paper, and SnO2 nanoparticles overlayed on film are further utilized to raise the sensitivity of sensor. The numerical SnO2 sensing models coated with SnO2 NPs of two different particle interval are constructed, and the sensitivity enhance by 24.5% and 55.1% respectively compare to that only with SnO2 film. In the following research experiments, layerby-layer method is firstly applied in deposition of SnO2 film and the enhancement in sensitivity caused by SnO2 NPs tallies with the theoretical predictions. Finally, this paper goes further into the principles of sensitizing combined with the simulation of electric field strength and the characteristics of nanoparticles. The sensor with controllable film and high repeatability possesses high sensitivity up to 5334 nm/RIU theoretical and 4704 nm/RIU experimental, which provides information for utilization of nanoparticles in LMR sensing.