In order to promote the comprehension and communication between the military and designer, the process of the on-orbit service activity is modeled based on the platform of Tau G2. Using the visual modeling language UML2.0 and an example of on-orbit assembly, the activity of on-orbit service is modeled commencing with the process, and the model is validated. Pay much attention to the top and the activity of on-orbit service is analyzed synthetically from the task requirement, system node, structure relation, process description, and state transformation. It is helpful for the related person to comprehend the activity as a whole and obtain the consistency of comprehension.