Abstract: Moisture is the important environmental factor influencing the growth of Mikania micrantha. But few studies have described its adaptive capacity of adjusting and controlling the growth environment, including soil moisture and air humidity. The present study was conducted to set up 2 pieces of moisture gradient: control, 30%-60% of water content of soil and 80%-90% relative humidity of air; treatment Ⅰ, 10%-20% of water content of soil and 20%-30% relative humidity of air. The growth and photosynthetic characters of M. micrantha was influenced remarkably in treatment Ⅱ. In water stress conditions, the maximum net photosynthetic rate was only 1.94 mmol.m-2.s-1 and the maximum quantum efficiency 64.91 mmol.mol-1 photons, obviously reduced as compared with control (8.63 mmol.m-2.s-1 and 211.34 mmol.m-2.s-1). With relative humidity of air used as one of the factors of water stress, this study reveals that relative air humidity could expand the growth range of M. micrantha, and air venting is the main reason that limits the growth of M. micrantha in the water stress condition.