To systematically research the flow law of supercritical steam in the wellbore and accurately calculate the heat transfer characteristics and thermal loss during the flow，we construct mathematical model to calculate the heat loss during supercritical steam flow in the vertical wellbore based on the principles of mass conservation，momentum conservation and energy conservation. The model comprehensively considers the physicochemical properties of supercritical steam and its heat transfer law during flow. Finally，a series of calculations are completed for the sensitivity analysis of parameters. The results demonstrate that the pressure of supercritical steam in the wellbore grows while the temperature decreases with a rise in well depth. When the steam injection rate reaches 12 t/h，the temperature along the wellbore reduces slightly and then rises at a well depth of 3 000 m. However，when the steam injection rate is 8 t/h，the temperature，after decreasing first，begins to increase at a well depth of about 850 m and then declines at about 2 300 m. At higher initial temperature of steam injection，the temperature drops faster while the pressure rises more swiftly along the wellbore. At a well depth of 1 000 m，the temperature drop is only 0.45% at the steam injection temperature of 400 ℃，while it is 5.17% at 450 ℃. In addition，during supercritical steam flow，phase transition will be observed at the over low steam injection rate or the over large wellbore depth. The depth of phase transition is about 1 000 m at the steam injection rate of 2 t/h，while it is about 2 150 m at 4 t/h.