Gnomon Shadow Measurement was a fundamental astronomical measurement in ancient China, being used for cosmological construction. The theory of Gaitian in the Zhoubi suanjing is based on the erroneous assumption that the sun's shadow changes “one cun for one thousand li” as the gnomon moves north and south. A new explanation has been proposed for the source of "one cun for one thousand li” hypothesis, explaining it with the observation of the sun with a sighting tube. Since the Han Dynasty, discussions on the structure and size of heaven and earth have become important research topics for astronomers. From the 3rd to the 6th century, although the theory of Huntian replaced the theory of Gaitian and became mainstream, the hypothesis of "one cun for one thousand li" was also questioned and challenged, this hypothesis was still used in the cosmological construction of the Huntian theory, betraying the contradictory attitude of astronomers on the issue of Gnomon shadow measurement. It was not until the Tang Dynasty when the astronomer Yi Xing conducted large-scale meridian measurements, linking the height of the North Pole with the length of the meridian, that the fallacy of "one cun for one thousand li" was finally eliminated.