I have studied the front two volumes of the Kaiyuan Zhanjing 開元占經 compiled by Gautama Siddha 瞿曇悉達 during the Kaiyuan era of Tang Dynasty. This book is well known to be a Chinese astrology encyclopedia. But the discussions on the Heavenly bodies were recorded in the front two volumes of the Kaiyuan Zhanjing are nearly unrelated to astrology. The following are the basic points of my findings. Firstly, the Kaiyuan Zhanjing picked up and followed strongly the Huntian 渾天 theory which viewed the heavens as a celestial sphere among various cosmological models. Secondly, Huntian theory was developed along with the advances in astronomy. The Chinese cosmologists began to discuss about the reliable size of the Heaven, the accurate distance from earth to sky and the method of making a better armillary sphere. Furthermore they had strong doubt about the shadow principle that the shadow of an eight-chi 尺 gnomon diminishes by one cun 寸 for every 1000 li 里, which used to be taken as an axiom to Chinese astronomer for a long time. Finally, we need to reconsider the history of Chinese astronomy. Many historians of Chinese science understood the Chinese astronomy was developed in mathematical and astrological field only, on the ground that Chinese astronomer did not take an interest in cosmology after Han Dynasty. But there were many cosmologists in China after Han. The Chinese cosmologists continuously discussed the Heavenly bodies and worked to find a better model and a convincing explanation for the Heaven for almost five hundreds years.