The effect of irrigation at different growth stages on root dynamic changes and yield of soybean in arid and semi-arid regions in Inner Mongolia was investigated with soybean cultivar Jiyu 47 as material.The results showed that irrigation at branching or flowering could significantly increase root fresh weight,enhance root activity,and enlarge root active absorption area.Irrigation during whole growth period increased root fresh weight and absorption area,decreased root activity;while no irrigation treatment had opposite results.On the condition of irrigated one time during whole growth stage,the highest yield of 2 305.5 kg·ha-1 was achieved when irrigated at branching stage.