Vitamin C is an acidic hexose derivative, which has two isomers of L-type (ascorbic acid (AA)) and D-type (dehydroascorbic acid (DHA)). DHA is the first stable oxidation product of AA and is the reversible oxidized form of AA. Therefore, any discussion of the nature and measurement of AA will involve the nature of DHA in the same system. The ultraviolet spectrum is a visual representation of how easy and difficult the electron transition is. Unreasonable theoretical calculation method and molecular model construction will lead to misjudgment of the maximum absorption peak of vitamin C, and thus can not accurately characterize the excitation properties of vitamin C. In order to accurately explore the antioxidant mechanism of vitamin C, based on density functional theory (DFT) and time-dependent density functional theory (TD-DFT), the molecular structure, ultraviolet spectrum and electron excitation characteristics of ascorbic acid (AA) and dehydroascorbic acid (DHA) of vitamin C were calculated and analyzed at the level of pbepbe/6-311++g(2d,2p) and B3LYP/6-311++g(2d,2p) in liquid phase environment in this paper. The results showed that the pbepbe/6-311++g(2d, 2p) method is the more accurate method to calculate the ultraviolet absorption spectrum of AA. Compared with AA, the ring structure of DHA has a significant plane distortion than AA. According to the analysis of the spectral contribution shows that the ground state transition to S1, S2, S3, S4, S14, S18 excited state is the main reason for AA ultraviolet spectrum, the absorption peak of AA at 200.171 5 nm contains the electronic excitations of n→π^* and n→σ^* electronic transitions, the absorption peak at 266.924 8 nm contains n→π^* and π→π^* transitions. The reason for the ultraviolet spectrum of DHA is mainly due to the ground state transition to S6, S9, S12, S13, S15, S16, S17, S19, S20 excited state, the strongest absorption peak of DHA is located at 181.024 8 nm, which has the transition characteristics of n→σ^* and n→π^*. The weak absorption peak at 231.346 39 nm refers to the n→π^* transition, and the absorption peak at 282.466 8 nm mainly corresponds to the n→π^* transition. By analysing the hole-electron distribution and its derivatives, it is possible to qualitatively identify the characteristics of the 7 excited states that play a major role in the AA absorption peak and the 9 excited states that make major contributions in the DHA absorption peak. Among them, the S4, S13, S14 excited states that make major contributions to the AA ultraviolet spectrum and the S6, S9, S17, S20 excited states that make major contributions to the DHA ultraviolet spectrum have obvious charge transfer, the centroid center of the hole and the center of the electron centroid are separated, which can be referred to as the charge transfer excitation, the separation of electrons and holes in other excited states is very small, which can be referred to as local excitation.