Most of the current prediction model of wellbore temperature for deep-water gas well does not consider the influence of natural convection in annulus on the heat dissipation of the system, resulting in a lower prediction accuracy of temperature. In this study, three-dimensional simulation on the heat transfer by natural convective of testing fluid in annulus was performed. The mechanism of heat transfer are studied for different values of Rayleigh number (Ra) and Bingham number (Bn). The results show that the occurrence of natural convection in the annulus can significantly increase the heat loss of the fluid in the tubing. With the increases in Ra or decreases in Bn, the convective transport in annulus gradually strengthens, and the heat transfer coefficient gradually increases. However, when the Bingham number increases to about 100, the heat transfer mode in annulus becomes a single heat conduction. Based on the simulation results, a new correlation of heat transfer coefficients in annulus was proposed. The introduction of this correlation can significantly improve the prediction accuracy of wellbore temperature during deep water gas well testing, and lay a foundation for the prevention and control of hydrate and wax formation in wellbore.