In order to measure the air drag coefficients for the circular plate， the cylinder and the streamline 　　body，a small wind tunnel of adjustable air velocity is designed. The height difference of U type tube in the water column is used to express the air velocity with method of orifice plate through a cantilever beam force sensor measurement air drag of test item. The air drag coefficient is determined according to the air resistance and the difference of the height of the water in U tube.