Abstract: In this study, a series of undrained pure principal stress rotation tests on intact clay are carried out by use of a hollow cylinder apparatus. The magnitudes of the mean principal stress, shear stress and intermediate principal stress coefficient are kept constant during the principal stress rotation. The influence of the intermediate principal stress on pore water pressure and deformation behavior is studied in particular. The mechanism of the effect of the principal stress rotation is analyzed. The test results show that the pore pressure and strain of intact clay accumulate with the principal stress rotation even when the three principal stresses are kept constant. The directions of the principal strain increment and the principal stress are not coaxial. The influence of the intermediate principal stress coefficient on pore pressure and strain accumulation as well as stiffness degradation is significant, but not obvious on the non-coaxial behavior. The mechanism of the effect of the principal stress rotation can be attributed to the disturbance of the microstructure of intact clay.