Abstract: In near-circular orbits, the external heat flux of satellites varies significantly over time due to the complex spatial relationships among the satellite, the Earth, and the sun. Conventional simulation methods were often computationally intensive and lacked intuitive interpretability. In this study, a geometric visualization-based method was proposed for the rapid analysis of the external heat flux of cuboid satellites in low Earth orbit (LEO). Transformation relationships were established among the geocentric inertial frame, the orbital frames, and the satellite body coordinate system. The trajectory of the sun vector was analytically modeled and dynamically visualized using GeoGebra. The heat flux on each satellite surface – including solar radiation, Earth’s infrared radiation, and albedo－was evaluated by projecting the sun vector onto the surface normals over a full orbital period. Compared to traditional numerical approaches, the proposed method provided improved computational efficiency and clearer physical insight. The method’s accuracy and engineering applicability were validated through comparison with Thermal Desktop simulations, making it suitable for early-stage thermal control design and configuration optimization in satellite constellations.