The cross sensitivity is a bottleneck for fiber grating sensor applications. Based on the sensing theory of fiber Bragg gratings, the physical mechanism of the cross sensitivity in the simultaneous measurement of strain, temperature and humidity is analyzed. The thinking of double grating method for solving the problem of cross sensitivity has been expanded by coating the humidity sensitive material and temperature sensitive material on the out player of fiber grating. The simultaneous measurement of strain, temperature and humidity has been realized by using the three grating method and the relevant de modulating scheme has been proposed. The high precision measurement of the wavelength shift of sensing grating can be realized by applying pressure and pull on the cantilever based match grating to match with the sensing fiber Bragg grating.