CSR and polycarbanate （PC） track detectors ha-ve been used for measuring fast neutron fluences ranging from 106 to 1011n/cm2. A comparative study of two different modes （the internal and external radiator） of fast neutron measurements based on SSNTDs is presented. Results show that the CSR plastic detector can be used successfully for detecting 14 MeV neutron fluences in the range from 5×107 to 6×1010n/cm2. The detection efficiency of the CSR is on the average 2.88×10-5 tracks per neutron and the CSR with polyethylene（PE） radiator is 3.32×10-5 tracks/a The detection efficiency of the PC detector with a thin layer of uranium is 1.75 × 10-6 tracks/n for detecting fast neutron fluences in the range from 2.5×107 to 5×1010n/cm2.