This paper describes an orthogonal signal demodulation method used in electromagnetic tomography (EMT) system. The method, which uses two phase adjustable orthogonal vectors, can demodulate the boundary magnetic signals of EMT sensor effectively and rapidly. To generate the excitation signal and orthogonal vectors, direct digital synthesizers (DDS) are used. And a novel correlation feed back algorithm is designed to adjust the initial phases of two orthogonal vectors. The image reconstruction experiment demonstrates that the demodulation method is effective.