Electrical heater is an important type of active thermal control methods. A dynamical equation of its control object's transient temperature change is established through principle analysis. The heating power demand calculation models at different temperature control models have been developed from the dynamical equation. The calculating flow of electrical heater's heat power has also been introduced. An active heating loop design example, which is included at the end of this paper, shows that the equations developed above can be used conveniently in satellite's active thermal control system designing and calculating.