In order to reveal the mechanism of hydrolysis and metal complexation of kaempferol, the UV-vis spectra of kaempferol in hydrolysis process (pH from 2.01 to 13.00) and in Cu(II) complexation process (from 0.1 to 2710.9 μmol/L) was obtained, and the characteristic spectra at various reaction stages was obtained by differential treatment. Combined with quantum chemical theoretical computation, a variety of possible deprotonation structures, complexation structures and corresponding electron absorption spectra were calculated based on DFT (Density Functional Theory) and TD-DFT (Time-Dependent Density Functional Theory). The experimental results are compared with the calculated values and it indicates that the hydrolysis process is in the deprotonation order of 7, 4′, 3, 5 phenolic group; at low concentrations of Cu(II) (<67.9 μmol/L), Cu(II) chelated with 4,5 phenolic group simultaneously; at high concentration of Cu(II) (67.9–2710.9 μmol/L), Cu(II) chelated with 3,4 phenolic group simultaneously. The calculated spectra is consistent with experimental spectra in hydrolysis and Cu(II) complexation process of kaempferol. The appearance of characteristic peaks is due to the change of the electron distribution of the active groups before and after hydrolysis and Cu(II) complexation, resulting in the changes of energy gap from HOMO (highest occupied molecular orbital) to LUMO (lowest unoccupied molecular orbital) during photon irradiation. The mechanism of hydrolysis and metal complexation process of kaempferol was revealed.