Abstract: The analysis of the external heat flux on launch orbit is extremely important for the thermal design of both upper stage and satellites. This paper proposes a method for the calculation of the vector to the Sun and the vector to the Earth based on a group of orbital and attitude parameters. The external heat flux for the cylindrical upper stage is analyzed. It is indicated that the solar illumination is the dominant external heat flux on the launch orbit. The vector to the Sun is relatively invariable during the long coast phase, while both the vector to the Sun and the percent time in the sunlight vary greatly with the launch time, which complicates the extreme cases. The sun-orientation and a slow spin around the longitudinal axis is recommended for the improvement of the thermal environment and the simplification of the thermal control system for satellites and upper stage.