The invention discloses a weak signal detection system and method for suppressing strong background noises by using optical path cancellation. The system comprises a laser, two couplers, a sensing fiber, a reference fiber, a fiber length tuning device, a first-stage photoelectric detecting device and a second-stage detecting device. The laser, the two couplers, the sensing fiber, the reference fiber, and the fiber length tuning device form an interferometer structure. The first-stage photoelectric detecting device detects the noise signal in an interference fringe and carries out feeding backand controlling on the fiber length tuning device, thereby suppressing the influence of the noise signal optically. The second-stage detecting device carries out high-precision detection on a residualsignal and is capable of measuring a weak signal covered by the large noise. According to the invention, the influence large noise signal is suppressed by using a method of optical closed-loop feedback controlling of the interference arm length and thus the weak signal detection is realized. The weak signal detection system and method have advantages of high sensitivity, large dynamic range and small noise influence.