We get the binary Taylor expansions of the effective refractive index and period of fiber Bragg grating (FBG), verify theoretically that the temperature-strain and strain-temperature cross sensitivities are the same, and make a conclusion that the thermo-optical coefficient and the elasto-optical coefficient are both proportional to the square of the effective refractive index. The calculation equations of temperature and strain are listed, and the method of calculation is expatiated. The wavelengths of FBG under different temperatures and strains are measured, and the cross sensitivity coefficients of temperature-strain and strain-temperature are -1.4743×10 -6 nm/(℃·με) and -1.3948×10 -6 nm/(με·℃). We place the FBG hydrostatic leveling instrument on the open-air balcony for one day, monitor the wavelength changes of FBG, and then calculate and get a more accurate solution to the changes of liquid depth in the hydrostatic level instrument. The experimental results show that using the calculation equations of temperature and strain can improve the measurement accuracy of FBG.