The invention provides a paramagnetic gas molecule concentration detection system and method. The system comprises a controller, an optical unit, an electric measurement unit, a feedback frequency locking unit and a cavity length locking unit, wherein the controller is sued for generating a laser modulation signal and a magnetic field modulation signal; the optical unit is sued for generating laser and converting gas molecule concentration information into polarization information of cavity transmission light; the electric measurement unit is used for generating an alternating magnetic field in a resonant cavity, converting the polarization information of the cavity transmission light into an electric signal and extracting a harmonic signal reflecting gas concentration information from theelectric signal; the feedback frequency locking unit is used for converting a reflection light signal from the harmonic cavity into an electric signal and extracting an error signal for adjusting laser driving current from the electric signal, so as to realize the locking of laser frequency and resonant cavity frequency; and the cavity length locking unit is used for controlling a piezoelectric ceramic driver to lock the cavity length of the resonant cavity. The method is capable of improving the detection sensitivity of paramagnetic gas molecules.