Abstract: The Saint Venant’s postulate is inapplicable to soil materials, namely there is a deviation between the major principal stress and strain increment directions. A series of tests with principal stress rotation are carried out on intact Hangzhou soft clay with ZJU-HCA. The stress paths under continuous rotation, cyclic rotation of principal stress and increasing principal stress direction angle and deviatoric stress are designed to study the non-coaxiality behaviors and influence factors. The non-coaxiality between the principal stress and strain increment directions is remarkable and quite different from that of sands. The degree of non-coaxiality in the tests with larger deviatoric stress or principal stress rotation with increasing deviatoric stress is smaller. In the cyclic principal stress rotation tests, the non-coaxiality is more complex when b≠0. The non-coaxiality is more significant when the samples experience reverse rotation of the principal stress. Additionally, the degree of non-coaxiality decreases as the development of shear strain and sometimes even negative values is investigated in non-coaxial angle.