Abstract: In order to on-line measure the relative neutron flux density for different space and energy in the reactor, a new scintillator fiber detect system was developed. The system consists of five kinds of detectors, five photon counters, a computer and the corresponding software. The main composition materials of five detectors are ⁶LiF+ZnS(Ag), ²³²ThO₂+ZnS(Ag), ²³⁸UO₂+ZnS(Ag), ⁹Be+ZnS(Ag) and BGO crystal, so the different energy relative neutron flux densities can be detected. The detector doped with ⁶LiF is used for thermal neutron measuring. BGO detector is used for gamma ray measuring. And the other three kinds ofdetectors are used for fast neutron measuring. The system was used for thermal and fast neutron relative flux density distributions measuring in Venus1# assembly. The measuring data were compared with the data gotten by Monte-Carlo method. The measuring data are considered to be credible considering the difference between the theory parameters and the real experiment parameters.