High-precision plumb bob for building

The utility model discloses a high-precision plumb bob for a building. The line pendant comprises an infrared emitter, a cover body and a line pendant head provided with a cavity and an emission hole,and is characterized in that the lower end of the infrared emitter is provided with an infrared emission port, the upper end of the infrared emitter is provided with a power switch, the power switchis sleeved with a spring, the infrared emitter is installed in the cavity, the infrared emission port corresponds to the emission hole, and the upper end of the cavity is provided with internal threads; external threads are arranged at the lower end of the cover body, an upper sliding cavity and a lower sliding cavity are formed in the axis of the cover body, the diameter of the upper sliding cavity is smaller than that of the lower sliding cavity, a piston is installed in the lower sliding cavity, an ejector pin penetrates through the upper sliding cavity, and the lower end of the ejector pinis fixedly connected with