Abstract: Emergent plants, an important component of urban wetland vegetation, absorb carbon dioxide and release oxygen via photosynthesis; these plants also regulate humidity through transpiration, playing an important role to improve well-beings of urban residents in high-altitude and arid areas． The physiological indexes and environmental parameters of reed (Phragmites australis) and cattails (Typha orientalis) in different places and different time periods were measured in this study, to dissect characteristics and influencing factors of their spatiotemporal changes． Net photosynthetic rate showed multiple peaks each day, with cattails (14.72 µmol·m⁻²·s⁻¹) higher than reed (8.46 µmol·m⁻²·s⁻¹)． Transpiration rate of cattails (7.68 mmol·m⁻²·s⁻¹) was higher than reed (3.90 mmol·m⁻²·s⁻¹)． Water-use efficiency showed unimodal diurnal variation, with reed (2.34 μmol·mmol⁻¹) higher than cattails (2.03 μmol·mmol⁻¹)． Net photosynthetic rate of reed and cattails was higher at inlet than at outlet． Transpiration rate of reed at inlet was higher than at outlet, cattails at outlet was higher than at inlet． Water-use efficiency of reed at inlet was lower than at outlet, but there was no significant spatial difference in water use efficiency for cattails． Photosynthetically active radiation, water vapor pressure difference, atmospheric temperature and water quality were important factors affecting the spatio-temporal changes of photosynthesis and transpiration of emergent plants． Carbon fixation and oxygen release capacity and humidifying ability of cattail were higher than reed． These data will support evaluating and improving ecosystem services of wetlands in Xining city．