Nuclear thermal propulsion (NTP) could be an advanced technology to facilitate a new and excellent rocket engine that would at least double the performance of the best conventional chemical rocket engines. NTP has been under development for several decades and was selected as the leading candidate technique for the manned mission to Mars, as suggested in Design Reference Architecture 5.0. During development, many concepts have been proposed, designed, and tested. Among which, the particle bed reactor (PBR) is the one of highest performance, and its compact and lightweight features make it ideal for space applications. In this paper, the thermal-hydraulic characteristics of a PBR are mainly investigated through two studies. The first study is to evaluate whether the principles derived from the PBR of uniform heat release could be applied in the cases of a nonuniform heating profile. The second study is to analyze the effects of some aspects, including porosity of the hot frit and cold frit, power shift, inlet temperature of the coolant on the internal flow, and heat transfer processes in the PBR of a nonuniform heat release. These findings may provide technical support for the subsequent design and optimization of the PBR.