<p>The interaction and feedback between 3D neutronics and thermal hydraulics are of great significance in reactor safety analyses, particularly for the TRIGA reactor. Owing to the TRIGA reactor’s pulse-transient operation status, the power changes by six to eight orders of magnitude within an extremely short duration; this operation is significantly different from PWRs and imposes some challenges for conventional neutronics methods. To describe the transient status of rod insertion or withdrawal, a novel time-dependent particle transport algorithm based on the combined and moving geometry methods is developed and integrated into the neutronics code MagicMC, which is a Monte Carlo particle transport code developed by the Nuclear Energy and Application Laboratory. Combined with the&#xa0;subchannel model, this work presents neutronics and thermal-hydraulics coupling methods for high-fidelity simulation of the TRIGA reactor. First, a steady-state coupling method is established based on over-relaxation iteration, and the number of neutrons in the Monte Carlo simulation is adaptively controlled according to convergence. Subsequently, a transient coupling method is proposed based on the semi-implicit coupling strategy, and a dynamically changing time-step strategy is designed for the coupling iterative process to achieve reasonable convergence. The parameter mapping strategy between neutronics and thermal hydraulics was constructed using one-to-one mapping and volume weight methods. To verify the reliability of the methods, a JSI TRIGA Mark II reactor was selected as the validation benchmark. The coupling results were in good agreement with the experimental data of the JSI TRIGA Mark II reactor, and the coupling methods achieved a high-fidelity numerical simulation of TRIGA reactor. Therefore, the coupling methods proposed in this paper can provide technical support for reactor experiments and the safe operation of the TRIGA reactor.</p>