This paper presents discontinuous zigzag gait analysis for a newly modeled quadruped walking robot with an articulated spine which connects the front and rear parts of the body. An articulated spine walking robot can move easily from side to side, which is an important feature to guarantee a larger gait stability margin than that of a conventional single rigid-body walking robot. First, we suggest a kinematic modeling of an articulated spine robot which has new parameters such as a waist-joint angle, a rotate angle of a front and rear body and describe characteristics of gait using an articulated spine. Next, we compared the difference of walking motion of newly modeled robot with that of a single rigid-body robot and analyzed the gait of an articulated spine robot using new parameters. On the basis of above result, we proposed a best walking motion with maximum stability margin. To show the effectiveness of proposed gait planning by simulation, firstly the fastest walking motion is identified based on the maximum stride, because the longer the stride, the faster the walking speed. Next, the gait stability margin variation of an articulated spine robot is compared according to the allowable waist-joint angle.