A flow blockage analysis model of a single channel is established using the best-estimate code RELAP5/MOD3.4. The reactor core is divided into seven hot channels, one average channel, one bypass channel, and corresponding fuel plates to take into account the interaction between the obstructed channel and adjacent channels. The coolant system is also modeled in detail to perform a better estimation. As a typical pool-type research reactor, JRR-3M is chosen for the analysis. The results indicate that the model can effectively simulate a single-channel blockage accident using the RELAP5/MOD3.4 code. Also, the thermal-hydraulic parameters in the blocked channel would be significantly affected if bubbles are generated as the blockage ratio continues to increase, which may damage the integrity of the fuel plate. Meanwhile, as for flow blockage of a single channel, the effect on adjacent channels is limited, even under high-blockage-ratio conditions.