The invention discloses a lower limb exoskeleton power assisting device driven by pneumatic muscles, including the control chamber, back device, lumbar device, lower devices and the foot device that are sequentially connected; lumbar device includes rotating member and elongate member, and rotating member provides an auxiliary degree of freedom and is provided with safe spacing gear button, change and coordinate the length that can adjust lumbar device between the hole of elongate member; lower devices are double to pulling structure using the pneumatic muscles of pulley wire cord type, by two pneumatic muscles antithesis pulling steel wire ropes, drive pulley rotation so as to realize joint drive using frictional force of the flexible wire ropes with pulley room; motion intention closed loop control each articulation of the control chamber according to operator, so as to drive leg exercise to reach power-assistance purpose. The invention has the advantages of simple structure, size-adjustable, flexible and reliable, stable and reliable, and the like, and can effectively improve the carrying capacity and the control accuracy of the power assisting device, and is convenient to wear, easy for maintenance, cost-saving, clean, and environmental protective.