In China by use of an erecting pole to measure the shadows of the sun has a long history as usually to derive the time of solar terms. We investigate three historical documents, the Zhoubi Suanjing, one of the most important ancient Chinese books on mathematical astronomy, was compiled about 100 BC and the Shoushi Liyi compiled in 1279, an ancient treatise for reviewing Shoushi Li calendar and the book Wushen Lichun Kaozheng compiled in 1608. Some conclusions are obtained that at latest in BC 560 the Chinese people had launched this method to measure the shadows of the sun, and suggested the incorrect law of ‘cun qian li’, which says the shadow length of a gnomon of 8 chi (about 1.96 m) high will increase (or decrease) 1 cun (1/10 chi) for every 1000 li (roughly 400 km) the gnomon moves northward (or southward). The ancient method to get the time of solar terms is not accurate enough, as also not supported by modern astronomy. The time of winter solstice obtained usually as a most fundamental data for every ancient calendar. The height of the pole had increased from 8 chi to 60 chi, but the accuracy is not increased enough, as usually not better than 1’.