Background: Detection efficient is one of the important technical parameters in the device of measuring radon in balloon method.Its value would influence the veracity of the result.Purpose: This paper had explored a new method about calculating detection efficient of the device measuring radon in balloon method.Methods: The expression of detection efficient has been deduced based on the principle of measuring radon and the new device which is IED-3000F developed by Chengdu University of Technology.Results: The detection efficient’ experimental value has been measured.Conclusions: The result show that this theory used in calculating detection of balloon method for measurement of radon is quite feasible in the range of error.