Abstract: Ultrasonic flowmeter is a device that can achieve online flow measurement. It is very important to develop an effective non-real flow calibration technique for ultrasonic flowmeter on the scene. The non-real flow calibration of the ultrasonic flowmeter is based on the calibration of the propagation time difference of ultrasonic wave and the calibration of geometric dimensions. Because of the limitations of local equipment and the need for easy operation, this paper was based on the geometric dimension calibration and tried to find the parameters that can impact on the result of flow measurement. Then, with the help of the experiments, the influence of the parameters on the flowmeter measuring results were confirmed. A process of non-real flow calibration for some DN80-type multipath ultrasonic flowmeter was used as an example to analyze and verify non-real flow calibration techniques for small diameter ultrasonic flowmeters.In the process of calibration, the pipe diameter was measured by the inside diameter micrometer, the sensor array of the flowmeter was used for the excitation and reception of ultrasonic wave, the correlation method was used to process the signals of ultrasonic wave, the ultrasonic path distances were calculated, and the factory parameters were correspondingly modified. Results show that the measurement error is reduced significantly after calibration. Therefore, the proposed method can be used for small diameter multipath ultrasonic flowmeter calibration.