Abstract: To study the effects of drought stress on chlorophyll contents and photosynthetic characteristics of cucumber seedlings, the author carried pot experiments by using cucumber seedlings as materials to research the effects of PEG imitation drought stress on them. The results showed that the decreasing amplitude of chlorophyll contents, gas exchange parameters and chlorophyll fluorescence characteristics of cucumber seedlings increased by improving drought stress concentration and prolonging stress time. The different of total chlorophyll content between compare (CK) and 5% PEG was indifferent, but their of 10% and 20% PEG treatment decreased by 5.2% and 16.3% respectively than CK at drought stress first day. The total chlorophyll content of 5%, 10% and 20% PEG treatment reduced by 8.6%, 12.0% and 37.2% respectively than CK at drought stress second day. The different of net photosynthetic rate(Pn), transpiration rate(E), stomatal conductance(Gs) between compare (CK) and 5% PEG was indifferent, while the different of intercellular concentrations of carbon dioxide (Ci) among treatments of CK, 5%, 10% and 20% PEG was different at drought stress first day, but the different of Pn, E and Ci among treatments of CK, 5%, 10% and 20% PEG was different at drought stress second day. At drought stress second day, the effective photochemical efficiency of PSII (YIELD) of 5% , 10% and 20% PEG treatment reduced by 2.4% , 8.7% and 13.2% respectively than CK, the photosynthetic electron transfer rate (ETR) reduced by 4.6%, 9.3% and 12.7%，while non-photochemical quenching of fluorescence but increased (qN) reduced by 8.7%, 18.1% and 66.6%.