Abstract: Using the B3LYP method of density functional theory, the author optimized ground state stable geometries of threonine (Thr) molecules and charged ions at the level of 6-311+G(2-df) basis set. The method of time dependent density functional theory  (TD-DFT) was used to study the excited states of the Thr molecule and ion system under the implicit solvent ethanol. The results show that with the increase of the number of electrons captured by molecules, the energy band of the molecule system decreases slightly. The S1—S10 excited states of Thr2- ion system are all obtained from the occupied orbital of MO33. The excitation characteristics of Rydberg are good, and the excitation mode of the excited stste of ion system which captures electrons changes significantly.

Key words: Thr molecule, ion, low excited state, orbital transition