A quasi-distributed fiber Bragg grating (FBG) demodulating system is developed by combining the wavelength tunable laser and the optical time domain reflectometry (OTDR) technique. By optimizing the optical pulse modulation and the light detection circuit, the ultra-weak reflective FBGs can be demodulated and located. In the experiment, 20 ultra-weak reflective FBGs of only 0.01% reflection with the interval space of 2 m are connected to the end of a 5.8 km long fiber. All the central wavelength and position of the FBGs can be measured simultaneously. The measured FBG center wavelength has a good linearity of 99.7%.