The invention relates to a method which realizes the encryption of SDH signals by making use of key bytes; wherein, the encryption of frame synchronization bytes is realized by randomly selecting a key from a key table by every N frames of a sending end and inserting a key number of the key into unused overhead bytes of a frame structure after the sending end and a receiving end realize the framesynchronization, and at the same time, the encryption operation for the present frame synchronization bytes is carried out by the key and operating results are inserted into the frame structure as newframe synchronization bytes and every N frames change a value of the frame synchronization bytes; a frame synchronization module of the receiving end continuously monitors an unused byte, and once the value changes, corresponding keys are found from the key table to make decryption operation for the value of the present frame synchronization bytes according to the value; the encryption of data communication channel encryption bytes is made before the SDH is framed and the decoding of the data communication channel encryption bytes is realized after the receiving end is decoded. The method avoids the problem brought about by direct encryption of high speed information payload, lowers down the requirements for the processing speed of hardware in the encryption operation and has good encryption effect.