Based on a triple-clad numerical model, a long-period fiber grating chemical sensor coated with the sol-gel gas-sensing films on the cladding of the fiber grating region is studied. The relation between the sensitivity Sn and the thin film optical parameters (thickness h3 and refractive index n3), and the fiber grating parameters (the grating period, the core index modulation and the grating length) is analyzed. By using optimization method, the optimal optical parameters of thin film and fiber grating parameters are obtained to make the sensor highly sensitive. Numerical simulation shows that the sensitivity of this scheme to refractive index of the films is 10-8. In the end a long-period fiber grating sensor sensitive to C2H5OH is fabricated, which proves the necessity of the structure optimization of the sensor.