UV irradiation intensity changes have great influence on carbon cycle in lake, which is one of the most important carbon pools in nature. To understand the effects of UV irradiation on allochthonous dissolved organic matter(DOM)in lakes, simulated photodegradation experiments were conducted. We extracted humic acid from soil surrounding an alpine Lake Tiancai as allochthonous DOM. During the photo-irradiation process, the order of decreasing level of a₄₄₀ and E₄/E₆ was 40 W group, followed by 20 W group and control group. Moreover, S_R, a₄₄₀ and E₄/E₆ were significantly correlated, suggesting that the concentration and relative molecular weight of humic acid decreased with increasing UV irradiation intensity. Five fluorescent components were identified through using EEM-PARAFAC models, included UV humic-like component (C1), UVA humic-like component (C2), tryptophan-like component (C3), UVC humic-like component (C4), tyrosine-like component (C5). The photodegradation rate and degree of four fluorescent components in 40 W group were greater than 20 W group, the order of photodegradation rate was C4 > C3 > C2 > C1 and the order of photodegradation degree was C2 > C4 > C3 > C1, indicating that fluorescent components response diversely to different UV irradiation intensities.