As the ground air data link system is the standard equipment of the commercial aircrafts, it should be deeply and thoroughly understood. Considering that there are only several corporations abroad who supply the ground air data link system and little internal relative technology is accumulated, a fundamental study on transmission signal and demodulating signal of the ground air data link system centering on the ARINC-618 protocol is done in this article. Moreover, some simulation equipments have been done on the basis of ACARS signal generated by the transient aircrafts.