The invention discloses a dimension marking demonstrating device, which comprises a casing, wherein a support plate is fixedly connected onto each of the left and right ends of the bottom of the casing through a support leg; a magnetic plate is embedded in the middle end of the bottom of each support pate; the bottom of each support plate is fixedly connected with a rubber plate; a center processing unit, an image collecting module, a marking judging module and a marking point picking module are sequentially and fixedly connected onto the left end of an inner cavity of the casing from top to bottom; a camera installing plate is fixedly connected onto the middle end of the top of the casing. Through the effects of a first sliding device, a second sliding device, a first electric telescopicrod, a first inclination angle sensor, a first radar distance measuring laser, a motor, a third sliding device, a second electric telescopic rod, a second inclination angle sensor and a second radar distance measuring laser, the marking requirements of people on the graph horizontal direction and vertical direction, the circular arc, the circular center, the diameter and the radius can be met; theteaching burden of teachers is reduced; meanwhile, the study interest of students is also improved.