With the development of information processing technology, DCT has been used more and more widely. But multidimensional DCT hasn’t unified matrix expressions, and operates using 1-D DCT to each dimension successively, which cannot embody the overall space character of multidimensional transform well. So, a new SDCT method was proposed in the paper. First, several matrix operation methods were defined. Then, the operation principle of 1-D DCT, 2-D DCT and 3-D SDCT was described in detail. The theoretical analysis shows that the SDCT makes DCT unified and succinct to express, easy to comprehend and convenient to operate.