A flexible two-wheeled self-balancing robot system and its motion control method include a main controller 1, a motion controller 2, an assistant controller 3, two servo drivers 4, sensors, input/output devices, two DC motors 5 and a power supply system; a camera is installed on truck's top board 11, a tilt-meter and a gyro are installed both in truck 6 and chassis 8. Truck 6 and chassis 8 are connected with an elastic joint in which is installed a two-arm torsion spring and a cylindrical spring; the main controller 1 is connected to the motion controller 2, the assistant controller 3 and input/output devices. The user commands received by the main controller 1 are immediate commands or stored commands; through main controller 1 running the motion behavior decision algorithm, the motion controller 2 running the motion balancing control algorithm, and the servo drives 4 controlling the torques of the motors 5, locomotion on plane and remote control of the robot can be implemented.