The invention belongs to a pneumatic energy-saving and environment-protecting device, and particularly relates to a zero energy consumption automatic vent device for a washroom. The zero energy consumption automatic vent device comprises a support saddle, a sleeve, a bearing, a transmission shaft, a pneumatic impeller mechanism and a driven impeller, and is characterized in that the zero energy consumption automatic vent device is mounted on the outer wall surface on a top wall of a vertical shaft used for exhausting peculiar smells or water vapor in a building; and the pneumatic impeller mechanism which comprises a pneumatic impeller, a wheel hub, an impeller rod and a fixing pin is arranged at the top end of the exposed upper shaft section of the transmission shaft. The zero energy consumption automatic vent device converts the wind energy into the mechanical energy, realizes the exhaust of the peculiar smells in the washroom by the transmission shaft which drives the driven impeller in the exhausting vertical shaft to rotate, so as to enable the air flow to ascend, save the electric energy consumed by using an electric exhaust fan, and realize the low-carbon and energy-saving life pattern; and the zero energy consumption automatic vent device has the characteristics of reasonable design, compact structure and the like, and is simple and practical to use, and convenient for installation, manufacture and use,. The zero energy consumption automatic vent device is widely applied to residential buildings, malls, hotels, public buildings, and the like.