The invention relates to a coding and decoding method and a device thereof. The invention primarily comprises the following steps: after a previous frame adopts a first encoding mode for encoding, and a current frame needs to adopt a second encoding mode for encoding, the encoded data of the length of the overlapped data in the previous frame is acquired according to the determined length of the overlapped encoded data of the current frame and the previous frame; subsequently, the encoded data acquired from the previous frame and the encoded data of the current frame are encoded by adopting the second encoding mode, thus acquiring the encoded results. A corresponding mode is adopted at the decoding end for decoding. Accordingly, in the invention, transform domain coding and decoding is completed without filter operation, thus simplifying the transform domain coding and decoding course under an entire mode switch, and facilitating the realization of software and hardware.