The concept of almost perfect autocorrelation sequen ce pairs is given.Some characters of the sequence pairs are discussed,and the ne cessary existent conditions of the sequence pairs are given.By use of the charac ters and the necessary condition of existence,the scope of searching the almost perfect autocorrelation sequence pairs is reduced.The Almost perfect autocorrela tion sequence pair that is of the same length as a known sequence pair can be constructed by using the characters and the necessary condition of existence.