Deglaciation has intensified in alpine regions with climate warming, causing increasingly intense paraglacial debris flows (PDFs). Examining the hydromechanical mechanism and initiation of PDFs is essential to disaster relief in downstream areas. However, the establishment of source initiation of PDFs and early warning criteria are hampered by a lack of in situ observations of PDFs; therefore, providing a PDF early warning is still challenging. In this study, on the basis of long-time series field monitoring of rainfall, temperature, soil moisture, and surface displacement in the Tianmo gully, southeast of the Tibetan Plateau, the initiation mechanism of PDFs and the evolutions of the effective saturation, matrix suction, and suction stress were analyzed by combining the hydromechanical coupling analysis method of a slope for a long-term sequence. The measured evidence for the formation and evolution process of slope stability in PDF source area was provided under the conditions of precipitation and ablation. Furthermore, through the inversion of the key slope stability parameters, the thresholds of the hydraulic factors for the provenance slope failure were established, which could be used for early warning of PDFs. This study provides theoretical reference and long-time field data for monitoring and early warning of PDFs in paraglacial hazard-prone areas under global warming sequences.