Remote sensing is an effective tool to obtain physiological information of vegetation on a large scale. However, most of the previous researches on plant spectroscopy assumes that light condition is stable during observation. Plants in the field, especially understory plants, are frequently exposed to a fluctuating light intensity due to cloud moving, sun tracking, sunflecks etc. Most of vegetation indices such as NDVI are insensitive to such fluctuation, but photochemical reflectance index (PRI) is expected to change under fluctuating conditions. PRI is a normalized vegetation index using the reflectance at 531 nm with the reflectance at 570 nm as a reference wavelength and can be used to assess non-photochemical quenching (NPQ). NPQ is a mechanism to dissipate excess light energy as heat to protect the photosystem II under stressful conditions such as high light and water deficit. NPQ is quickly activated though biochemical processes such as de-epoxidation of xanthophyll, but its change is generally slower than changes in light intensity under fluctuating light. Our study aimed to reveal temporal dependence of PRI and NPQ under fluctuating light and how they are coupling in the transition stage. This study used three types of poplar with different stomatal changes to identify physiological mechanisms of temporal changes in NPQ and PRI. We measured their gas exchange rates, chlorophyll fluorescence and reflectance spectrum in changing light. We calculated static indicators, such as initial photosynthetic rate, saturated photosynthetic rate, etc., and dynamic indicators, such as photosynthesis increase rate, half-saturation time, photosynthesis recovery rate, etc. We found that there are no significant differences in static indicators between three types of poplar, but there are significant differences in dynamic indicators between them. We found that even in changing light environment, PRI still maintains a high correlation with NPQ. PRI and NPQ respond to changing light much faster than photosynthesis. For dark-treated leaves, the photosynthetic rate takes about half an hour to reach saturation, but NPQ and PRI only take a few minutes. We conducted correlation analysis on a wide range of spectrum, and the result showed that PRI is the vegetation index with the highest correlation with NPQ under changing light. This study shows that even if the static physiological parameters are same, there can be large differences in the total photosynthetic capacity of the ecosystem due to differences in dynamic physiological parameters. In dynamic light environments, PRI is a suitable index to estimate NPQ.