The invention relates to a method for reducing harmful components by a mechanical device; the method is used for eliminating harmful components in kiln tail flue gas when in cement production; the kiln tail flue gas produced in a rotary kiln (1) sequentially passes through a kiln tail smoke chamber (2) and a decomposing furnace (4) and are exhausted out from downstream equipment; the method is characterized in that: the kiln tail flue gas is contacted with a mechanical device (3), the temperature on the surface of the mechanical device (3), which is beneficial to condensing the harmful components, is formed, so as to lead the harmful components to be condensed on the mechanical device (3) and lead the condensed harmful components to be discharged to outside. The mechanical device can be arranged at the downstream of the rotary kiln and can be arranged in the downstream equipment of the kiln tail smoke chamber (2), or decomposing furnace (4) or a preheater system according to the performance of the harmful components needed to be discharged. In the invention, the mechanical device and the kiln tail flue gas are contacted, the temperature on the surface of the mechanical device (3),which is beneficial to condensing the harmful components, is formed, so as to lead the harmful components to be condensed on the mechanical device, and then the condensed harmful components are discharged into the system by taking measures.