In order to study the effects of different relative humidity on the leaf photosynthesis characteristic in a greenhouse, an experiment was conducted with eight models of relative humidity on cucumber Jinyou 1, which was chosen as the trail material. The photosynthetic physiology parameters of cucumber leaves were measured with the LI 6400 portable photosynthesis system. The results show that the net photosynthetic rate and intercellular CO2 concentration reduced with the decreasing relative humidity; the stomata conductance, transpiration rate, and the leaf water use efficiency reached the maximum when relative humidity was 75%, while the stomatal limitation value reached the minimum when relative humidity was 75%. The relationship of maximum photosynthetic rate and air relative humidity conforms to the exponent equation: 〖WTBX〗P〖WTBZ〗max=223753e189085fRH. Through the light response curves, it can be learned that the apparent photosynthetic quantum yield and light saturation decrease with deceasing relative humidity, and the light compensation point increases with the decreasing relative humidity. In addition, the correlative analysis indicates that a negative correlation between the vapor pressure deficit of leaves and relative humidity, and a positive correlation exists between intercellular CO2 concentration, stomata conductance transpiration rate, relative humidity, and photosynthetic rate.