Abstract: Traditional equation of steam quality is improved based on definition. Rate of wellbore heat loss is calculated with both depth step and time step. Drift-flux model of two-phase flow by Hasan is used to estimate pressure drop in wellbore. It indicates that results of modified model and new algorithm agree well with field date. They are more accurate than Beggs-Brill method. In addition, steam pressure, temperature and quality are almost unchanged at the same depth. However, rate of wellbore heat loss decreases with injection time. The algorithm provides a reference for accurate calculation of wellbore heat loss.