Accurately predicting the wellbore annulus ECD is a prerequisite for safe and efficient drilling. To enhance its calculation accuracy, a calculation model of drilling fluid density is obtained by using multiple linear regression method based on the rheological test data of water-based drilling fluid under high temperature and high pressure, and a calculation model of ECD in the annulus of small borehole at high temperature and high pressure is further established. The results show that the higher the ground temperature gradient increases, the higher the annulus temperature grows and the smaller the annulus ECD decreases; the smaller the annulus size reduces, the larger the annulus ECD increases; the presence of cement line leads to the lower temperature and the larger the annulus pressure consumption, which in turn leads to the larger annulus ECD.