The invention discloses a device and a construction method for preventing an insulating layer of a building external wall from falling off. The device comprises base plates and hooks, wherein the base plates are respectively in an elliptical shape, the working surfaces of the base plates are respectively and fixedly connected with the hooks, and an included angle between each hook and the corresponding base plate is 60 degrees; the base plates are connected with a waterproof layer of the external wall by an adhesive; the hooks extend into the insulating layer of the external wall. According to the device and the construction method, the manufactured hooks are stuck to the waterproof layer of the external wall at uniform intervals in virtue of the powerful adhesive, and the hooks are painted in a thermal insulation material. Therefore, the insulating layer of the external wall is effectively prevented from falling off from the waterproof layer, so that the construction quality is guaranteed, and the working efficiency is greatly improved.