The invention discloses a tree diameter at breast height automatic measurer, which relates to a measurement device, particularly relates to a wireless sensor network-based tree diameter at breast height automatic measurer and belongs to the technical field of tree measurement tools. The tree diameter at breast height automatic measurer comprises a binding band; the binding band is a resistance band; one end of the resistance band is provided with a fixed knot and the other end of the resistance band is provided with a mobile knot; the fixed knot is arranged in a fixed knot box, and a first buckle is arranged on the side surface of the fixed knot box; the mobile knot is arranged in a spring pressure box, a second buckle is arranged on the side surface of the spring pressure box, the first buckle and the second buckle are located in the same horizontal plane, and the first buckle and the second buckle are connected via a clamping band; a fixed knot wire and a mobile knot wire are respectively connected onto the fixed knot and the mobile knot, and tail ends of the fixed knot and the mobile knot are sealed in the spring pressure box; tail ends of the fixed knot wire and the mobile knot wire are bent; and the spring pressure box is internally provided with a measurement control unit. The measurement reading is accurate, the efficiency is high, and the safety is good.