Fiber grating is an optical passive device which has been greatly developed in recent years. The largest application for fiber Bragg grating (FBG) is the fiber sensor. Cross-sensitivity of fiber grating sensor is the most important problem which has restricted the development of the fiber sensor. In this paper, we explain how the crosssensitivity problem is produced based on the basic principle, and we suggest a modification for the traditional dual-wavelength matrix calculation method, which is used to solve the cross-sensitivity problem. The modified calculation method has a higher accuracy than the traditional one.