Abstract: Photosynthetic characteristics were studied in mature leaves of Phalaris arundimacea using portable LI-6400 photosynthesis instrument(Licor,USA) in the field.The diurnal changes of net photosynthetic rate and its major affecting factors,including photosynthetically active radiation(Par),stomatal conductance(Cond),intercellar CO₂ concentration(Ci),leaves temperature(Tl),air temperature(Ta),transpiration rate(Tr),were measured.The results showed that the diurnal process of photosynthetic rate exhibited a double peak curve in which the main peak appeared around(10:00),and the second around(15:00).This may reflected an obvious phenomena of midday depression of photosynthesis caused by Non-stomatal limition.Diurnal variation of net photosynthetic rate is 5.86-20.64 μmol·m^-2·s^-1,and transpiration rate is 2.8-6.22 mmol·m^-2·s^-1.And the other parameters,light compensation point is 40 μmol·m^-2·s^-1,light saturation point 2100 μmol·m^-2·s^-1,apparent quantum yield 0.0565 mol·mol^-1,dark respiration rate 2.26 μmol·m^-2·s^-1,and water use efficiency is 2.31-3.65 μmol·mmol^-1.The data showed that P.arundimacea was a kind of highly photosynthetic efficiency plant,illuminating for the exploitation of resources and maintaining the eco-stability of wetland ecosystem.