Abstract: The cross-sensitivity is a key problem in the application of fiber grating sensors. Based on the sensing theory of fiber Bragg gratings (FBG), the physical mechanism of the cross sensitivity in simultaneous strain and temperature measurement with fiber grating is analyzed. The FBG reflected wavelength equation with the temperature-strain cross-sensitivity coefficient is established. The error analysis of the equation was made with the dual-wavelength matrix algorithm and the relative error curves of temperature and strain were obtained in the case of cross-sensitivity. The effects of the cross-sensitivity on the measurement were discussed and analyzed according to the expressions and curve diagram of the relative error. The result shows that the relative errors of temperature or strain grow significantly with the increase of temperature or strain variance during the measurement of temperature and stress, and the crosssensitivity is not negligible when the relative errors are great enough.