Organic matter content (w_u) and water content (ω) greatly influence the mechanical properties and engineering characteristics of peat and peaty soil (peat soil for short). However, the determination standards of ω and w_u of soil with high organic matter content are unclear in existing codes, procedures, and standards, and the test results vary greatly among different units. To define the determination standards of ω and w_u of high organic content soil, tests were performed under different burning temperatures and burning times. Generally, the site of lacustrine sedimentary soil in Dianchi Lake has 3-5 layers of peat soil. To explore the variation law and evolution mechanism of stress and strain under different buried depths and confining pressures, a series of triaxial consolidation undrained (CU) tests was performed on five layers of peat soil with different buried depths under low to high confining pressures. The test results show that the stress-strain curve of peat soil presents a “layered” phenomenon with increasing confining pressure; the strain corresponding to the maximum deviatoric stress decreases with increasing depth, and the curve shape changes with the confining pressure and buried depth of soil samples; the pore pressure coefficient decreases with increasing depth. Moreover, the control standard of failure strain of peat soil in the CU test is proposed. This research further deepens the understanding of the mechanical properties of peat soil and provides an experimental basis for determining ω and w_u standards.