A new gas blow forming-cold contraction bonding process for forming Mg/Al composite tube was presented .The circumferential flow stress and elongation of AZ31 magnesium alloy and 7475 aluminum alloy were obtained by the unidirectional hot tensile tests at 420℃, 440℃and 460℃. On the basis of these datas, the initial gas pressure and maximum gas pressure required to deform the Al tube and Mg tube was calculated and the AZ31/7475 composite tube was successfully manufactured at 460℃. The as-formed AZ31/7475 composite tube is closely combined with no metallurgical bonding. Due to the existence of eccentricity between the AZ31/7475 tubes and the die cavity, the wall thickness distribution of the cross section is not uniform; The residual contact stress between outer tube and inner tube was deduced, which includes the residual contact stress caused by the elastic recovery and the residual contact stress caused by the cold shrinkage of the two alloys. The residual contact stress of the composite tube was measured by the compression tests and the calculated value agrees well with the measured value, with a difference of about 19.2%.