This paper describes a collision avoidance method of a humanoid robot based on controlling the extended orbital energy. When a robot detects an obstacle using an arm tip, it should stop short of the obstacle to avoid crushing. However, modification of the ankle torque may destabilize a walking motion. We propose a method that the robot stops by utilizing an arm force. The COG trajectory is modified during walking by controlling the arm force. The robot stops within 2 steps after contact between the arm tip and the obstacle. During the first step, the COG velocity is reduced by changing the arm force. Here, the arm force is decided based on the extended orbital energy. During the second step, the robot stops just above the supporting point. In addition, the arm force becomes 0 smoothly when the robot stops. In the result, the robot is easily able to switch to the next action after stop.