Abstract: The conceptual design of a space lithium cooled fast neutron reactor was analyzed with MCNP. The nuclear reactor used uranium mono-nitride as the fuel, lithium as the coolant, and tungsten-25% rhenium as main structural materials. The reactor was controlled by drums in the side reflector. The core calculation models based on MCNP were made. By calculations and analyses, the main dimensions and physics parameters of the core were given. The reactivity worth of the control drums, core burnup and power distributions were calculated. The Re’s spectral shift absorb (SSA) character that ensures reactor’s subcriticality when fully submerged in wet sand or water, following a launch abort accident and the 7 a full power running no need refueling were validated.