Gait planning of humanoid robot is a hotspot at robotic field. The problem that swing arms have an effect on gait during the walking and running is analyzed in this paper. Standing on the view of energy, a special pendulum model established on the motion of swing arms is used to analyze the effect about swing and angular velocity of arms to gait planning. The experiment indicates, the optimal swing angle and the optimal swing speed facilitate the stability and validity of gait planning in the humanoid robot motion.