Dissolved organic matter (DOM) in lakes is ubiquitous and plays an important role in the aqueous ecosystem. DOM in sub-alpine lakes is dominated by the terrestrially-derived humic acid in the soil. Photodegradation is an important transformation process of DOM because of its strong light absorption, changing the form of carbon, nitrogen, phosphorus. Under the scenario of climate warming, DOM concentration, pH, dissolved oxygen and solar radiation in subalpine lakes may change. However, the effects of these environmental changes on DOM photodegradation in sub-alpine lakes are still unclear. In this paper, the photodegradation of allochthonous DOM in a sub-alpine lake-Lake Tiancai (located 200 m below the tree line) were studied. Firstly, the humic acid (THA) was extracted from the soil around Lake Tiancai. Then the photodegradation of THA was investigated at different light bands (ultraviolet or visible light) and different pH conditions. The results showed that four components were identified by excitation-emission matrix fluorescence coupled with parallel factor analysis (EEM-PARAFAC), including C1-C3 were humic-like fluorescence, accounting for 83.64% of the total fluorescence intensity, and C4 was protein-like substance, accounting for 16.36%. Under the irradiation of 500 W mercury lamp, the molecular weight of THA decreased gradually and DOC concentration decreased by 44.5% after 10 h. The value of a₂₈₀ (a proxy for colored dissolved organic matter (CDOM)) decreased gradually with the irradiation time; while the fluorescent dissolved organic matter (FDOM) showed an overall trend of increasing at first and then decreasing. Compared with the weakly alkaline media (pH=8.0), the weakly acidic media (pH=6.0) facilitated for the photochemical process of THA and acidification may further promote DOM degradation. Oxygen played a crucial role in DOM photochemical reaction as the pseudo-first-order kinetic constant of a₂₈₀ in the presence of oxygen was 3.25 times that in the absence of oxygen. The ultraviolet light was much more efficient for THA degradation than visible light. The apparent quantum degradation ratios (η) of ultraviolet light were about 5-11 times that of visible light. This result of this study suggested that under the scenario of climate change, the photodegradation of DOM in sub-alpine lakes might accelerate, enhancing the role of sub-alpine lakes in the carbon cycle.