A method for constrained power allocation in a multiple input multiple output (MIMO) system, including: executing a singular value decomposition (SVD) operation upon a channel matrix; obtaining a diagonal matrix, wherein the diagonal matrix is composed of a plurality of local matrixes; and transforming diagonal elements of the diagonal matrix into diagonal elements of a specific diagonal vector from an upper-left corner local matrix and a lower-right corner local matrix, in parallel, simultaneously, and according to a sequence, by using a generalized triangular decomposition.