Abstract: Many types of neutron detectors are resonant in keV region, and the cross section is not accurately known. Therefore, it can’t be used as primary standard detector for neutron fluence rate measurement in keV region. The cross section of ⁶Li is accurately known in this energy region, and the LiF-SSD detector system was designed which consists of ⁶LiF conversion and Si semiconductor detector. The primary standard detector of neutron fluence rate measurement in keV region was established. The programs of SRIM and TRIM were used to evaluate the count rate of the LiF-SSD detector. The response of LiF-SSD detector calculation program was developed based on Geant4. The pulse height distribution of 27.4 keV neutron was calculated. Based on the reaction of ⁴⁵Sc(p, n)-⁴⁵Ti, the pulse height distribution of 27.4 keV neutron was measured, and it is close to calculation result.