Based on coupled mode theory,a theoretical model is established for longperiod fiber thin film sensors.The sensing characteristic is analyzed and gassensing experiment with preliminary experiment results is presented.The results are demonstrated to show that the thin film parameters(thickness and refractive index) have direct relation to the sensitivity of sensors and the structure optimization is very important.With the advantages of both film sensors and fiber optical sensors,the longperiod fiber thin film sensors will have a wide application prospect in industrial area.