Abstract: The discrete element simulation of the triaxial test of unsaturated soil is crucial to reveal its mechanical properties from a mesoscopic perspective,however,a rigid boundary condition has been adopted in most existing studies.In this paper,by applying equal effects to the membrane particles,the flexible loading of confining pressure to samples was simulated.On this basis,a numerical simulation method for the triaxial test of unsaturated soil is proposed,and the triaxial numerical tests under different confining pressures are performed.The mechanical characteristics of unsaturated soil under different confining pressures are studied from the aspects of macro failure mode,mesoscopic shear band,coordination number,and contact force.The results show that the mechanical properties of unsaturated soil are greatly affected by confining pressure.The peak stress and elastic modulus increase with confining pressure.The greater the confining pressure,the smaller the dilatancy.The deformation is mainly characterized by bulging deformation.The shear band presents a symmetrical “X” in shape after deformation.The formation of the shear band begins with the rotation of the particles.When the peak stress is reached,the internal rotating particles are connected,and the shear band is basically formed.The larger the confining pressure,the smaller the number and rotation angle of rotating particles on the shear band.The coordination number first increases and then decreases,which is consistent with the changing trend of volumetric strain and increases with the increase of confining pressure.The peak value of probability density function of normal contact force moves to the left and the non-uniformity of normal contact force increases due to the shear of sample.The greater the confining pressure,the more uniform the distribution of normal contact force is.The research results can provide a reference for understanding the mechanical behavior of unsaturated soil from a mesoscopic perspective.