The invention relates to a high-precision optical fiber grating sensing signal demodulator. A tunable laser source is disposed at the left upper part of a mounting plate, a 1*N light splitting coupler and a N-path optical coupler are disposed at the right upper part of the mounting plate, the output end of the light splitting coupler is connected with the N-path optical coupler with an optical fiber and outputted, a data acquisition card, a multi-channel detector and a gas absorption cell are disposed in the lower part of the mounting plate, one output end of a first path optical coupler is connected with a first input end of the multi-channel detector via an optical fiber, the other output end of the first path optical coupler is connected with the input end of the gas absorption cell via an optical fiber, the output end of the gas absorption cell is connected with a second input end of the multi-channel detector via an optical fiber, one output end of a second path optical coupler is connected with a third input end of the multi-channel detector via an optical fiber, the other input end of the second path optical coupler is connected with the fourth input end of the multi-channel detector via an optical fiber, the input ends of other paths of optical couplers are connected with other input ends of the multi-channel detector via optical fibers, the multi-channel detector is connected with the data acquisition card via a cable, a computer is connected with the data acquisition card, a filter and the multi-channel detector.