The invention discloses a flexible spine mechanism. The flexible spine mechanism comprises a bionic spine, pneumatic bionic muscles and supporting baffles; the bionic spine comprises an elastic plateand a plurality of bionic vertebrae different in size, the bionic vertebrae are sequentially and fixedly arranged on the elastic plate from large to small in the length direction of the elastic plate,the end, with the large size, of each bionic vertebra is the waist of the bionic spine, and the end, with the small size, of each bionic vertebra is the chest of the bionic spine; the supporting baffle plates are divided into a front support baffle plate and a rear support baffle plate; the front supporting baffle is fixedly connected with the chest of the bionic spine, and the rear supporting baffle is fixedly connected with the waist of the bionic spine; and the pneumatic bionic muscles are arranged on the two sides of the bionic spine, and the two ends of the pneumatic bionic muscles are fixedly arranged on the front supporting baffle and the rear supporting baffle correspondingly. The rigidity-variable kangaroo-like hopping robot flexible spine mechanism is simple in structure and used for solving the technical problems that at present, a hopping robot is insufficient in trunk flexibility, invariable in spine rigidity and complex in structure.