The quantity and quality of soil organic matter (SOM) in the wetlands and peatlands are important for evaluating of the effects of environmental changes. This study’s aim was to evaluate the change in the chemical properties of SOM and dissolved organic matter (DOM) during a constant temperature incubation of high-moor peat soil under two types of vegetation. Incubation of high-moor peat soils collected from marsh vegetation and dwarf bamboo was conducted for 108 days at each temperature of 25 °C and 35 °C. The chemical properties of alkaline extract and DOM in soil samples during incubation were analyzed by tetramethylammonium hydroxide thermochemolysis-gas chromatography-mass spectrometry (TMAH-GC/MS) and fluorescence analysis. The cumulative CO2 emission from peat under dwarf bamboo was higher than that of peat under marsh vegetation. During incubation at 35 °C, plant and microbial residues in DOM extracted from dwarf bamboo soil were increased significantly at the early stages of culture. On the other hand, the components of DOM in the marsh vegetation soil sample did not significantly change between incubation at each temperature. The fluorescence spectra showed that protein-like fluorescent DOM contained in dwarf bamboo soil is consumed by microorganisms, which promotes leaching of humic-like fluorescent DOM and carbon mineralization during the incubation period at a higher temperature. Compared with marsh vegetation soil, the DOM in dwarf bamboo soil is susceptible to temperature rises and can be a larger source of CO2 emissions. This study shows that evaluation of DOM properties in soil could be useful to assess the effect of climate change on soil environment.