Abstract: In this paper,motion in Z-direction of the mass centre of the swing leg of biped robot is planned into aparabola,its inertial force balances the weight of swing leg and half weight of the body.The ZMP is kept atX-direction linear through support foot centre,thus the biped robot in walking will not fall down in flank di-rection.Distribution method of parallel coordinate frames is adopted so that the inverse solution is simplified.