Abstract: In this study, three sites of each E-W and S-N oriented viaducts in hanghai were selected, and the light condition and its diurnal variation under the viaducts were measured in the four seasons of a year. The special characteristics of light condition were analyzed by combining the indexes of photosynthetic photon flux, light compensation point, and light requirement level, and the results showed that the light condition and its diurnal variation changed with different viaducts, and even, under the same viaduct. The main affecting factors are the quantity of viaduct, viaduct height, width between viaduct deck and green belt, and weather condition. The heterogeneity of light condition under viaduct brought out the “dead position” and “excessive light’s damaging position” of plant grow th. It was suggested that suitable greening distribution could be effective in i mproving the growth condition of the plants under viaduct

Key words: Davidia involucrata, Population, Population density, Proteting measures