A novel fiber Bragg grating (FBG) sensor system with an active closed cavity is proposed according to the principle of fiber loop mirror (FLM) based on 3 dB wideband coupler fabricated by fused biconical taper technique. Ten cascaded wavelength-division-multiplexed (WDM) FBG sensors are fabricated in the FLM acting as an end mirror of a closed cavity resonator. Active interrogation techniques are realized by controlling the applied voltage of the insert tunable Fabry-Pérot filter of the cavity and realizing wavelength scanning. The unbalanced Michelson interferometer transforms the wavelength-shift signal induced by the measured strain into phase-shift signal, and the demodulation is achieved. The experimental sensor sensitivity, 1.5835 °/10-6ε matches the predicted value (1.6662 °/10-6) basically.