Abstract: This paper presents a method for measuring the reciprocity parameter based on the free field. It is able to achieve accurate measurement of the reverberation constant in a narrow band. The method uses the same transmitting and receiving system, and keeps the same set of parameters to measure the open circuit voltage output under different frequencies in a free field. The open circuit output voltage is measured through average technology in the reverberation control region, then the reverberation radius is calculated and the reciprocity constant obtained. This method uses a single frequency signal and the spatial averaging technique. It is simple, convenient and not suitable for complex measuring instruments. The validity of the method is verified by comparing the measured results with the reverberation time measurement.