Objective: To analyze the differences on photosynthetic characteristics between tetraploid and diploid Polygonum capitatum. Method: LI-6400XT portable photosynthesis system was used to measure various photosynthetic physiological parameters of tetraploid and diploid P. capitatum and their variation regularities. Result: The diurnal variation of net photosynthetic rate (Pn) of tetraploid and diploid P.capitatum s leaves showed double-peak curves with the phenomenon of ‘midday depression’

but the Pn of tetraploid was higher than that of diploid.Both P.capitatum cultivars' diurnal variation of stomatal conduction (Gs) and transpiration rate (Tr) were consistent with that of Pn

and intercellular CO2 concentration (Ci) diurnal variation trend were exactly opposite to that of Pn.Gs

Tr

and water use efficiency (WUE) of tetraploid were higher than that of diploid

but Ci of tetraploid was lower than that of diploid.Light compensation point (LCP) of diploid P.capitatum [67.83 μmol·(m2·s)-1] was higher than that of tetraploid P.capitatum [41.43 μmol·(m2·s)-1]

and light saturation point (LSP) of diploid P.capitatum [1225 μmol·(m2·s)-1] was lower than that of tetraploid P.capitatum [1370 μmol·(m2·s)-1].The maximum net photosynthetic rate (Pnmax) of diploid P.capitatum [5.36 μmol·(m2·s)-1] was lower than that of tetraploid P.capitatum [8.83 μmol·(m2·s)-1]

and apparent quantum efficiency (AQY) of diploid P.capitatum [0.01 μmolphotons·(m2·s)-1] was lower than that of tetraploid P.capitatum[0.02 μmolphotons·(m2·s)-1]. Conclusion: Photosynthetic performance of tetraploid P.capitatum was superior to that of diploid P.capitatum.