Abstract: To accurately identify the features of annular multiphase flow in high temperature high pressure (HTHP) conditions, a fully transient temperature and pressure coupling model for multiphase flow in deep or ultra-deep wells was established according to the theory of wellbore multiphase flow and heat transfer. Considering the changes of physical parameters of circulating fluid with temperature and pressure, and the iterative algorithm was proposed. In this paper, a deep well in Tarim Oilfield was analyzed for the wellbore transient temperature and pressure coupling. The results indicated that the drilling fluid density at bottomhole increased from 1 360 kg/m3 to 1 460 kg/m3, and the plastic viscosity increased from 8. 6 mPa·s to 13.8 mPa·s after circulating for 8 hours. Both the drilling fluid density and the bottomhole plastic viscosity increased over the circulating time, and further, the bottomhole pressure dropped linearly at initial circulation and then to the minimum at 0.2 h. Later, it increased logarithmically and tended to be finally stable. In view of the impact on bottomhole pressure, the factors are were prioritized in a descending order as follows:gas influx, surface drilling fluid density, displacement, wellhead back pressure, and surface drilling fluid plastic viscosity. An analysis of the results could provide certain theoretical guidance for the hydraulic parameter design of deep and ultra-deep wells.