The cowpea seedlings were treated when they had four leaves and a heart by water stress.The results showed that the soil water content and leaves water content continued to decrease,the stomata opening rate and stomata conductance continued to decrease too,and they were different from not treated significantly when stressed 8 days later,the chlorophyll content decreased and was different from not treatment significantly when stressed 8 days later，the intercellular CO2 concentration firstly was stable，then decreased,finally continued to increase，and was different from not treated significantly when stressed 8 days later,the transpiration rate and photosynthetic rate decreased continuously，the transpiration rate had remarkable difference from not treated when has been stressed for 4 days,while significant difference when has been stressed for 6 days,the photosynthetic rate had remarkable difference from not treated when has been stressed for 6 days,while significant difference when has been stressed for 8 days.The photosynthetic rate was positively related to stomata opening rate,stomata conductance and chlorophyll content,while was negatively related to intercellular CO2 concentration.It was concluded that stomata factor was the main factor effecting photosynthetic rate.The cowpea seedlings can endure water stressing for 8 days.