Abstract: The scintillator fiber detector used two probes to identify the neutron signal. The detector system was tested by ²⁵²Cf fission source and was compared with the ³He counter. Meanwhile, the detector was used in Venus 1# assembly to measure the thermal neutron relative flux density distribution. The relative efficiency of the detector was calibrated with the neutron conversion rate from Geant4 code and neutron spectrum from MCNPX code. The results were compared with the fission rate measured by solid-state nuclear track detector. The measurement results show that the response of the scintillator fiber detector to the ²⁵²Cf neutron source agrees well with the attenuation of the point source, which is in good agreement with the counting rate of ³He counter. The relative distribution of the thermal neutron relative flux density measured at the Venus 1# thermal zone is consistent with fission rate distribution by the solid-state nuclear track detector. The same result of fast region is in line with the counting rate distribution measured by ³He counter and simulated by MCNPX code. The results provide important experimental data for the development of the detector.