Abstract: The moderator temperature effect is positive of the particle bed reactor（PBR） model of SNTP, which will have a bad influence on the core safety. The changes of neutron spectra of reactor core, neutron production rates, neutron absorption rates and leakage rates at different ⁷LiH moderator temperature were analyzed with the method of neutron balance by using MCNP code based on the PBR model. The results show that when ⁷LiH temperature rises, neutron losing rates, including neutron absorption rates and leakage rates, increases obvious larger than that for neutron production rates, which caused PBR core presents positive moderator temperature effect, especially when it at lower temperature.