Abstract: A brief and accurate measurement method of active radiating factor of high-frequency waveguides of loud- speaker line arrays is studied. Based on the principle of multiplication and Fresnel principle, the theory illuminates the directivity response of line array is equal to that of the array element, which has a sinc function relationship with the active radiating factor. Experiments are designed based on the theory, and the results show that the active radiating factor is 82.5% and the mean square error of verification experiments between the calculated value and measured value is just 0.051. In conclusion, this method can obtain the active radiating factor as well as the directivity of the whole line array rapidly by single-pointed measurement of the directivity of array element.