Abstract: In order to investigate the physiological effects of nighttime high temperature on soybean（Glycine max（L.） Merr.） from sowing to flowering and determine whether phenotypic variation caused by long-term nighttime high temperature was caused by genotype differences, 9 soybean varieties were subjected to extreme nighttime high temperature（EHNT,30 ℃/30 ℃ day/night） from sowing to flowering to study the effects of this environment on physiological indexes and vegetative growth of soybean. The measures included photosynthetic rate, respiration rate, chlorophyll content and fluorescence analysis. The results showed that the long-term extreme night high temperature at high light intensity 1 750 μmol/（m~2·s） decreased the photosynthetic rate, stomatal conductance, chlorophyll content and PSⅡ efficiency（F_v/F_m）, but did not change the respiration rate. Varietal variation was observed from a significant decrease in these physiological indicators. Some species showed tolerance to extreme nighttime temperatures through unchanged aboveground biomass. In soybean varieties S14-15146R and DS25-1, extreme night temperature tolerance could be related to photochemical efficiency（F_v/F_m） of PSⅡ.