Abstract: When the outer diameter of the small diameter pipe is measured by the double ultrasonic probe immersed in water, the axis of the probe group cannot be completely coincident, and the imeasurable deviation of the rotation center of the probe group and the axis of the tube to be detected shall result in the error of the outside diameter being beyond the nuclear level of small diameter pipe measurement requirements. To this end, the discrete signal cycle separation and statistics of small pipe diameter measurement method were established, which shall improve the nuclear grade small diameter pipe diameter measurement accuracy without demanding mechanical installation accuracy. And the validity of the method was verified by the self-developed TESC-200 pipe high-speed detection system under the condition of 4 000 r·min^-1.