The purpose of this article is to conduct a comprehensive investigation on the onboard datalink system,and makes analysis of the main system architectures. With these process , this article forms a support on the onboard datalink system design. The investigation of onboard datalink system covers the system development history,design implementations, applicable regulations and standards. In addition, this article introduces CNS/ATM conception in which the technologies associated to the onboard datalink are focused on. At last, the article demonstrates the consideration of the key points about civil aircraft onboard datalink system design.