This paper carried out a detailed study on the Cretaceous fossil carbonate microfacies of Gamba. Twelve types of microfacies (MF1-MF12) and seven types of biofacies had been recognized. On the basis of these, the research made a preliminary discussion on the environmental evolution of Tibetan Tethys during Cretaceous, which could be concluded as follows: a transgression took place during the Barremian-Aptian. In the early of Albian, Gamba area was generally in a regressive environment. From the Late Albian to Cenomanian, the seawater had a deepening trend.There was a big transgression which took place during the Turonian and lasted to Santonian-Coniacian. The seawater became shallow rapidly during Maastrichtian.