Based on the coupled-mode theory,the complex characteristic equation of LPFG coated with weak absorption double-layer films is set up.By use of the perturbation method,the complex root is obtained.The result is coincident with the result of D.V.Ignacio.The film thickness and refractive index have a great influence on the resonant wavelength,but the extinction coefficient has a little influence on the resonant wavelength.And,the two methods of optimizing the film parameters are presented in order to obtain the larger shifts of the resonant wavelength.The results show that when the film parameter combinations are h3=122.76 nm,h4=400 nm and n3=1.572 2,h4=400 nm,the wavelength shifts are 10.35 nm and 11.74 nm,respectively,which is much larger than the value of 2.78 nm obtained from the LPFG coated single film.It proves that the selection of the thicker film increase the sensitivity.顾铮天|zhengtiangu@163.com