Abstract: Peoples in many countries believed that plant spirits looked like animals,which was a worldwide folk belief. In ancient China,there was a same folk belief:the tree spirit was green cow. Why did this folk belief exist? Maybe there were three aspects. Firstly,tree gods in myths looked like animals. Secondly,an important ceremony in Qin,Han Dynasties was using green cows made of clay to greet spring. In this ceremony,the cow was connected with spring and trees. Thirdly,the faith about green cows had a long history in ancient China,especially in Wei,Jin,Northem and Southem Dynasties,so it was very important.