This invention relates to a single wheel robot system and its control method. The robot is an intelligent self-control and thus self-balancing unicycle riding robot. The control method is the balance control method of the static imbalance unicycle robot. The single wheel robot includes mechanical body and control system; the body contains a single wheel in the substructure which can rotate around for balance; the control system comprises state sensors, motion controller, servo-driven controllers, and a power system. Among them, the motion controller receive signals from the state sensors, in accordance with control procedures for processing of the received signal, thereby issuing control instructions. The servo drive controller receives the control instructions and controls the motors of the robot to adjust posture to be balanced.