The rapid development of fiber grating sensing technology in recent years puts forward higher request on the multiplexing capacity and spatial resolution. The demodulation of quasi-distributed fiber Bragg grating (FBG)sensing with high capacity and high spatial resolution is achieved by adopting ultra-weak FBGs and optical frequency domain reflection technology (OFDR). In order to achieve the grating demodulation, the optimization of beat frequency signal separation and nonlinear correction to realize the extraction of fiber grating position information with high spatial resolution as well as the separation of the grating beat signal in the time domain are used respectively. Then the grating reflection spectral information by using the Hill Hilbert transform is restored. The system multiplex 200 FBGs along a single optical fiber have the same center wavelength of 1552.8 nm, reflectivity of 0.1% , space interval of 20 mm. The experimental results shows that, in the temperature range of -10 ℃～80 ℃, the measured center wavelength of FBG has good linearity of 99.6%.