In order to improve the accuracy of wavelength shift measurement which is limited by the scanning wavelength step in the traditional measurement, a method which combinates a coarse measurement and a fine one was present based on the crossspectrum detection and crosscorrelation technology. The coarse wavelength shift measurement which is based on the principle of crosscorrelation detection has achieved a wide range of measurement, and the fine wavelength shift mesurement which is based on the principle of crossspectrum detection has achieved a highprecision measurement. Both the range and the precision are improved with the combination of the coarse and the fine measrurement. The smoothing tecnlogy is applied for data processing, which has eliminated the influence of random noise in the detection system. The fiber grating wavelength shift is measured directly to reduce the system error which has avoided the measurement of the absolute values of FBG center wavelength. Simulation results show that the method breaks the limit of the scanning wavelength step, and the measurement accuracy is improved about three orders. FBG longitudinal load measuring experiment with the load range of 0~60 g was carried out and the new method was applied to process the experiment data, the results show that the FBG longitudinal load sensitivy is 13.2 pm/g, the new method has a better applicability.