In order to investigate the effect of elevated CO2 concentration on photosynthetic acclimation of cucumber,an open toptype gas chamber(OTC) was used.Two CO2 concentrations(380 μmol·mol1,1 000 μmol·mol1) and two K concentrations(200 and 400 mg·kg1) were applied to study the variation of photosynthetic characteristics of cucumber.The results showed that elevated CO2 concentration significantly increased the weight of fresh cucumber.And there were some significant interactions between CO2 concentration and K concentration on the weight.Cucumber growing in TOC with elevated CO2 concentration had photosynthetic acclimation,which occurred during 24～30 d.However,there was the potential to weaken photosynthetic acclimation with increased K concentration.The value of Fv/Fm of cucumber leaves was more stable with elevated CO2 concentration,and the values of Fo,Fm and Fv′/Fm′ were significantly increased with the increase of Pn during shortterm(14 d) elevated CO2 concentration.During the longterm(35 d) elevated CO2 concentration,there was no significant change of Fo and Fm,but the value of Fv′/Fm′ decreased.The increase of K concentration did not change the phenomenon caused by elevated CO2 concentration.At 35 day for photosynthetic acclimation,no sign of increased K concentrations was found to mitigate the decrease of photosynthetic parameters resulting from the photosynthetic response curve and the CO2 response curve.