Advancements in space exploration technologies have garnered increasing global attention, with various nations initiating programs encompassing lunar missions, deep space exploration, and Mars expeditions. Nuclear thermal propulsion (NTP) rocket engines, serving as energy supply systems for space activities, offer significant advantages over traditional chemical fuel and solar electric propulsion by achieving high specific impulse and thrust with minimal nuclear fuel consumption. Among various NTP system cycles, the closed expander cycle demonstrates superior engine performance, making it particularly suitable for deep space missions. This study employs the Modelica language and the OpenModelica platform to design and simulate a closed expander cycle-based NTP system. Mathematical models for the propellant and key components of the propulsion system are established, followed by the construction of a simulation model. Comparative analyses of the simulation results are conducted to validate the accuracy of the model, and dynamic simulations of orbital maneuvers are performed. Addressing the current challenges in China's NTP field, such as its late inception and significant gaps in simulation technologies, this research provides a reference for the modeling and simulation design of space nuclear thermal propulsion systems by integrating system modeling of the propulsion module using Modelica.