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 is hinged with a rear end face of the front running deck by a hinge, and a hinging gap is located at a location where the front running deck is hinged to the rear running deck; an annular running belt, which is rotatably mounted around the front running deck and the rear running deck with a clearance; and a flexible layer, which is laid on an upper surface of the front running deck and an upper surface of the rear running deck and covers the hinging gap.