The consolidated undrained triaxial shear and scanning electron microscope tests were conducted on five layers of undisturbed peat soil with different buried depths from lacustrine sediments of Dianchi Lake，and the shear characteristics and microstructural changes of peat soil under different confining pressures were analyzed. Test results show that under the confining pressure of 50-1800 kPa，the common tangent of the limit stress of peaty soil shall be drawn in sec-tions rather than in a straight line. The effective stress path curve of peat soil changes from disordered to ordered with the increase in confining pressure. When the consolidation pressure is larger than 300 kPa, the effective stress path curve is in the shape of " 7 ," and the consolidated drainage flow increases linearly under the confining pressure of 50-300 kPa. The mechanism of the consolidation shear process was analyzed in accordance with the microstructural change and macro shear process. Under the action of confining pressure, soil particles produce displacement and the contact mode of particles changes from edge-surface to surface-surface. Large pores in the soil are squeezed into small pores, especially those with a diameter of 1 μm. The essence of soil deformation lies in the change in soil particle position. With the increase in confining pressure, the orientation of pores is improved, and the distribution of soil particles is uniform. The shear strength and related parameters of peat soil increase when its density changes from loose to relatively dense and finally to dense.