Abstract: In this work, we firstly elucidated the ultraviolet light protection dynamics mechanism of the typical hemicyanines, i.e.. Hemicy and DHemicy, by combining the theoretical calculation method and the transient absorption spectra. It is theoretically and experimentally demonstrated that both Hemicy and DHemicy have strong absorption in UVC (200-280 nm), UVB (280-300 nm), and UVA (320-400 nm) regions. Moreover, after absorbing energy, Hemicy and DHemicy can jump into the excited states. Subsequently, Hemicy and DHemicy relax to \mathrmS_0 states from \mathrmS_1 states rapidly by the non-adiabatic transition at the conical intersection point between the potential energy curves of \mathrmS_1 and \mathrmS_0 states, and are accompanied by the trans-cis photoisomerism. The transient absorption spectra show that trans-cis photoisomerization occur within a few picoseconds. Thus, the ultraviolet energy absorbed by Hemicy and DHemicy could be relaxed ultrafastly by the non-adiabatic trans-cis photoisomerization processes.