This paper presents an emergency stop algorithm of a walking humanoid robot. There are many cases which force a walking robot to stop quickly without falling. Since an emergency occurs at unpredictable timing and at any state of robot, the stopping motion must be generated in real-time. To overcome these problems, our emergency stop motion is divided into four phases according to the role of the zero-moment point (ZMP). In each phase, approximate analytical solutions of the center of gravity (COG) dynamics is used to generate the motion. During the single support phase, a landing time and position are determined by evaluating the average velocity of the swing leg and the horizontal position of the COG. During the double support phase, the travel distance of the COG and the ZMP are evaluated. The validity of the proposed method is confirmed by simulation and experiment using a humanoid robot HRP-2.