A new method called critical suction is used based on the wall climbing robot demands of miniature structure, moving smartly and low noise.It makes the robot achieve the homeostasis state in the suction cup, and in this condition the robot can stay on the wall reliably and move smartly.The fluid mechanics model and fluid network model are set up to analyze the robot suction system when the airflow is steady or changes suddenly.Furthermore, simulation results indicate the close relation between the key parameters of robot structure and the suction system.Finally the method of critical suction proves correct in theory.