Nano-satellite＇s thermal system is fundamental for its passive thermal control. The analysis and modeling of its in-orbit dynamical performance are necessary for nano-satellite＇s thermal control scheme design and evaluation. A three-dimension nonlinear dynamical model of satellite＇s temperature change was built up by dividing the typical passive thermal control system of nano- satellite into three representative functional node, the in-orbit temperature changes of satellite surface and indoor environment can be calculated conveniently from this model. A set of equations for the in-orbit calculations of nano-satellite＇s external space heat flow and flight position were also introduced. These equations are based on nano-satellite＇s body-reference frame and share same solving algorithm with the three-dimension nonlinear dynamical model. These provide a new transient analytical model and simplified in-orbit calculating method for the developing of advanced nano-satellite thermal control and thermal management technologies. The in-orbit dynamical performance of a sunsynchronous orbit nano-satellite thermal system had been calculated as an example. The results and its discussion were described at end of this article.