The invention discloses a method and device for measuring the emanation medium radon exhalation rate in an open-loop mode. According to the method, the measurement of the radon exhalation rate is carried out by using the device. The device comprises a radon measuring instrument and a test column used for containing a to-be-tested emanation medium, a gas collecting cover is arranged above the testcolumn, the gas inlet of the radon measuring instrument is communicated with the gas collecting cover through an air pipe, the gas outlet of the radon measuring instrument is communicated with an external environment, a hole communicated with the external environment is formed in the upper surface of the gas collecting cover, a gas pressure equalizing chamber is arranged below the test column, thegas pressure equalizing chamber is connected with a gas pump through the gas pipe, and a flow regulator is arranged on the gas pipe between the gas pressure equalizing chamber and the gas pump. The method has the advantages that the free diffusion of the radon on the surface of the medium is considered, and the influence of the seepage effect is also considered. In addition, the device can accurately measure the radon exhalation rate of the surface of the medium in a short time.