In this work, we firstly elucidated the ultraviolet light protection dynamics mechanism of the typical hemicyanines of Hemicy and DHemicy by combining the theoretical calculation method and the transient absorption spectra. It was theoretically and experimentally demonstrated that both of Hemicy and DHemicy have strong absorption in UVC, UVB, and UVA regions. Moreover, after absorbing energy, Hemicy and DHemicy can jump into the excited states. Subsequently, the Hemicy and DHemicy will relax to S0 states from S1 states rapidly by the non-adiabatic transition at the conical intersection (CI) point between the potential energy curves of S1 and S0 states and accompany by the trans-cis photoisomerism. The transient absorption spectra showed that the trans-cis photoisomerization will occur within a few picoseconds. Thus, the ultraviolet energy absorbed by Hemicy and DHemicy could be relaxed ultrafast by the non-adiabatic trans-cis photoisomerization processes.