The invention discloses a lower limb rehabilitation exoskeleton system based on pneumatic muscle, including an exoskeleton mechanism, a support mechanism, and a rehabilitation training treadmill system. The exoskeletal mechanisms includes a lumbar support, a hip single joint mechanism, a single joint structure of the knee and a plantar fixation. The monoarticular structure including a base, a joint turntable, wire ropes, pneumatic muscles, a support part and a spring, wherein the support part is mainly composed of upper and lower support rods which are slidably connected. The support part is vertically arranged on the base; a pneumatic muscle and a spring are fixed on the base of the support part; the pneumatic muscle and the spring are fixed on the base; the output end of the pneumatic muscle is connected upward through a steel wire rope and the top end of the spring; and the steel wire rope wraps around the joint rotary table. The invention adopts pneumatic muscle to drive to complete periodic movement, achieves the purpose of rehabilitation training, has clean, pollution-free, friendly to people, and the pneumatic muscle is a driving element with certain flexibility, which accords with ergonomics.