In order to investigate the molecular structure and spectrum of Hesperetin (HES), a kind of dihydroflavone drug. Density Functional theory (DFT) and basis set 6-311G(d,p) are combined to optimize the ground state’s geometry of HES molecules under an external electric field (EEF) ranging from -0.005 to 0.010 a. u. in C2—C1 direction. Total molecular energy, infrared spectrum (IR), dipole moment (DM), and HOMO-LUMO Energy Gap are investigated at the same time. Based on the optimized configuration, time-dependent density functional (TDDFT) is applied to study the influence of different EEF on excited states and UV-Vis spectrum of HES. The results show that when there is no external EEF, the single bonds between C1—O18 and C2—C26 are optimized as double bonds, and the enol structure is converted to a more conjugated system, forming the most stable structure. With the increase of EEF, the total energy of molecule rises at first and then falls, DM decreases at first and then increases. The change of bond length is complicated. When the negative EEF increases, due to the IR absorption peak (AP) generated by the vibration of different chemical bonds in HES molecule, different spectrum shifts occur, and the intensity of each AP also varies according to its relative chemical bond. In the absence of EEF, there are two APs at 223.6 and 262 nm in UV-Vis, related to in the E2 band and the K band respectively. AP at 223.6 nm appears blue-shift (BS) with the increase of EEF. When EEF is greater than 0.002 5 a. u., peak 223.6 nm disappear. AP at 262 nm shows a red-shift (RS), and the absorption intensity shows a declining trend with the increase of EEF. When EEF is 0.01 a. u., peak 262 nm shifts to 283 nm, and it’s intensity reaches 5 898.64 L·mol-1·cm-1. BS occurs under negative EEF and the absorption intensity increases. When EEF is -0.002 5 a. u., peak 262 nm shifts to 261 nm and the intensity increases to 12 500.36 L·mol-1·cm-1. When positive EEF increases, both energy gap and excited energy (EE) show a decreasing trend, indicating that the HES molecule is easily excited and is in an active state. In the absence of EEF, oscillator strength (OS) is greater than zero, indicating that it can be stimulated. When EEF is continuously enhanced toward the positive direction, OS of ES increases at first and then decreases; OS has complex changes under negative EEF. Investigating the molecular structure and spectrum of HES under EEF will provide a theoretical reference for its electric field dissociation.