MPEG Surround (MPS) coding is an efficient method for multichannel audio coding. In an MPS encoder, downmixing from multichannel signals into a less number of channels is an efficient way to achieve high compression rate. In decoder, an upmixing module combining with the decorrelator is the key module to reconstruct the multichannel signals. This paper considers the design of the downmixer and the decorrelator with the assistance of the Gram-Schmidt orthogonal process. The performance of the proposed downmixer and decorrelator is verified through objective and subjective quality measure.