The invention discloses a sensing system based on optical OFDM (Orthogonal Frequency Division Multiplexing) and an FBG (Fiber Bragg Grating) monitoring method thereof, relating to the field of optical fiber sensing. The sensing system comprises an adjustable optical source, an electrooptical modulator, an optical circulator, a weak reflective FBG array, a wavelength tuning control circuit, a signal generator, an optical detector and a digital decoding and control unit, wherein the adjustable optical source is used as a signal optical source; the electrooptical modulator is used for modulating data onto signal optical waves; the optical circulator is used for separating optical signals which are in an incident direction from optical signals which are in a reflective direction; the weak reflective FBG array is used as a sensing element for reflecting the optical signals to the optical circulator; the wavelength tuning control circuit is used for tuning the emitting wavelength of the adjustable optical source; the signal generator is used for generating electric signals which are encoded based on OFDM; the optical detector is used for converting the reflective optical signals into the electric signals; and the digital decoding and control unit is used for positioning an FBG sensor and acquiring variables to be detected. In the invention, by combining the FBG monitoring method based on the optical OFDM, the arrangement density and the sensing distance of FBG sensors are improved, the scanning frequency and the spatial revolution of the system are improved, and large-capacity measurement requirements are satisfied.