A waist protective device includes a first support member, a second support member, two fixing seats each mounted on the second support member, two threaded rods each secured on the respective fixing seat, two mounting seats each mounted on the first support member, and two rotation members each rotatably mounted on the respective mounting seat and each having an inner portion provided with an inner threading screwed onto the respective threaded rod to move the respective threaded rod relative to each of the rotation members by rotation of each of the rotation members relative to the respective threaded rod. Thus, the second support member is movable relative to the first support member to change the relative position between the second support member and the first support member.