Continuous climate warming in northeastern China has intensified the melting of permafrost, leading to its degradation. The thawing of permafrost increases the instability of wetland soil organic matter and accelerates decomposition; this will inevitably have a strong impact on the transformation of soil organic matter and the process of vegetation growth. Therefore, the present study used the Xing’an Mountains in Northeast China as the research area. We selected typical marsh areas and analyzed the thickness of the active layer and seasonal frozen soil at different degrees of permafrost degradation. Soil and plant samples (herbs: Carex schumidtii; shrubs: Vaccinium uliginosum; trees: Betula papyrifera) from foliage, roots, and stems were collected at different depths to determine the carbon and nitrogen stable isotope contents in soil and plants. Analysis of variance and multiple linear regression were used to examine the impact of frozen soil degradation on soil organic matter turnover and vegetation growth. The results showed that the thickness of the active layer is increasing, and the amount of seasonal frozen soil is decreasing in the Xing'an Mountains. The δ¹³C, δ¹⁵N, ɛ_δ13C, and ɛ_soil¹⁵N values of different depths of permafrost were significantly different (P < 0.05). The foliage, roots, and stems of C. schumidtii, V. uliginosum, and B. papyrifera under different degrees of permafrost degradation were significantly different (P < 0.05), except for the δ¹³C value of B. papyrifera roots. Soil isotopic composition, and therefore SOM quality, was dependent on the isotopic composition of incoming biomass (foliage, roots, and stems).