Abstract: Miniature neutron source reactor (MNSR) is a typical pool-tank reactor, in which the natural convection is the major heat transfer method. In order to study the safety of MNSR, the transient thermal-hydraulic parameters under the rated power condition and accident condition were simulated. As to the rated power condition, the CFD software was used to conduct three-dimensional transient thermal-hydraulic numerical simulation while the RELAP5 code was used to calculate one-dimensional transient thermal-hydraulic parameters. The results of the two calculations are consistent, indicating the correctness of the calculation results and the safety of the rated power condition. The RELAP5 code was used to simulate reactivity insertion accident and output the transient thermal-hydraulic parameters after the accident, which further determines the dynamic self-stability characteristics and inherent safety of MNSR.