Abstract: In this paper, an improved method of constructing multiple quasi-orthogonal spreading codes and a new modulation and demodulation scheme based on position mapping are proposed on the research of methods of constructing multiple quasi-orthogonal codes, using new method of codes grouping and position mapping, and making the demodulating of the quasi-orthogonal branches into the combination of demodulating of orthogonal sequences codes, that will obtain better demodulation performance. Analysis and simulation prove that this scheme can reduce obviously the computer complexity in the receiver side. It will increase transmission speed of information and reduce bit error probability.