Dolby Lab’s AC-3 standard has been widely used in many fields. As an indispensable module, the implementation of MDCT/ IMDCT should be optimized in efficiency and structure. There is a variable parameter in the MDCT definition. Many fast algorithms have been presented to implement MDCT, however the difference among the computational structures of different types of MDCT brings inefficiency in code structure. Here a new algorithm on short MDCT using long MDCT ’s computational structure is proposed.