Nonphotochemical quenching (NPQ) provides an essential photoprotection in plants, assuring safe dissipation of excess energy as heat under high light. Although excitation energy transfer (EET) between chlorophyll (Chl) and carotenoid (Car) molecules plays an important role in NPQ, detailed information on the EET quenching mechanism under in vivo conditions, including the triggering mechanism and activation dynamics, is very limited. Here, we observed EET between the Chl Q(y) state and the Car SI state in highlight-exposed spinach thylakoid membranes. The kinetic and spectral analyses using transient absorption (TA) spectroscopy revealed that the Car Si excited state absorption (ESA) signal after Chl excitation has a maximum absorption peak around 540 nm and a lifetime of 8 ps. Snapshot TA spectroscopy at multiple time delays allowed us to track the Car S-1 ESA signal as the thylakoid membranes were exposed to various light conditions. The obtained snapshots indicate that maximum Car S-1 ESA signal quickly rose and slightly dropped during the initial high-light exposure (<3 min) and then gradually increased with a time constant of 5 min after prolonged light exposure. This suggests the involvement of both rapidly activated and slowly activated mechanisms for EET quenching. 1,4-Dithiothreitol (DTT) and 3,3'-dithiobis(sulfosuccinimidyl propionate) (DTSSP) chemical treatments further support that the Car SI ESA signal (or the EET quenching mechanism) is primarily dependent on the accumulation of zeaxanthin and partially dependent on the reorganization of membrane proteins, perhaps due to the pH-sensing protein photosystem II subunit S.