Abstract: In order to investigate the reproductive strategy and plasticity of reproductive allocation of the climbing, dioecious herb Humulus scandens under environmental stress, we established plants of one variety under a gradient of 3 water stress levels (light, moderate and severe), and explored its plasticity of reproductive growth. Measurements were performed for both female and male H. scandens, and included among others:flower bud differentiation, biomass allocation, spike traits, quantity and activity of pollen and anthers, and ovary characteristics. The results showed: 1) H. scandens responded to water deficit stress by earlier differentiation of flower buds, and the timing of floral initiation in male plants was earlier than that of female plants (P<0.01). 2) Male plant responded to light water deficit stress by increasing reproductive allocation, but female plants adapted to water shortage by decreasing flower biomass. Reproductive allocation of male plants was significantly larger than that of female plants (P<0.05), with a nearly threefold difference under severe stress. 3) Both female and male H. scandens responded to water deficit stress by decreasing the number of spikes. Under light and moderate water deficit stress, male plants significantly increased floret number per spike when adapting to environmental stress (P<0.05), while female plants decreased flower number and hence would have a lower energy cost of reproduction. 4) Under light and moderate water deficit stress, there was no significant difference in thickness of the female plant ovary. Under moderate and severe water stress, male plants had significantly shortened anther length and reduced pollen grain numbers per anther (P<0.05), but the pollen size and pollen activity was unaffected. 5) Floral POD and SOD activity was significantly higher in male plants than that female plants, possibly indicating that male plants are more sensitive to water stress. In summary, H. scandens, exhibits a response to water deficit stress involving earlier flowering and differences between male and female plants in investment in reproductive effort.