A running deck assembly and a treadmill are provided. The running deck assembly includes: a front running deck; a rear running deck, wherein a front end face of the rear running deck abuts a rear end face of the front running deck; a flexible layer, laid on an upper surface of the front running deck and an upper surface of the rear running deck; and an annular running belt, rotatably mounted around the front running deck and the rear running deck with a clearance.