The invention discloses a mountain truncating land making device and a construction method, and belongs to the technology of mountain truncating land making. The mountain truncating land making device comprises an opened conveying belt, a closed conveying belt, concrete piers, standard joints and climbable conveying belt supports. A site is dressed in a mountaintop excavation area to contain the opened conveying belt. The concrete piers are built in a mountain slope and a filling area, the standard joints are fixed to the concrete piers through bolts, and the climbable conveying belt supports are fixed to the tops of the standard joints. The closed conveying belt is arranged on the climbable conveying belt supports, connected with electric wires and powered on, the height of the climbable conveying belt supports is adjusted, a guide line is formed to connect the excavation area with the filling area, and the tail end of the guide line is sleeved with a nylon cloth bag. The mountain truncating land making device and the construction method are suitable for expansion of urban land space with the terrain being canyons and basins; the construction period can be shortened; the project cost can be reduced, and environment pollution can be reduced; unconsolidated differential settlement of a backfill area can be avoided; and hilly city development can be accelerated.