It is difficult to carry out the overall design for on-orbit service vehicle (OSV) because of its multiple functions, complex structure and various categories. To solve the problem, the depth technical analysis should be carried out. According to the demand of servicing mission procedure, the main function of OSV is analyzed, and the key technology needing breakthrough is put forward. Based on the different function of subsystems, the structure of OSV is analyzed, which is divided into two parts: platform and load. The OSV classification is investigated according to the diversification and mission period characteristic. The function demand, key technology, component structure and classification of the OSV derived from this article provide necessary technical support, and point out a proper direction for the macro plan and gross design of the OSV system.