Abstract: The ultra-deep slim-hole wells in the Shunbei No.1 Area have high wellbore temperatures. In some wells, the temperatures even exceed the temperature resistance of the existing domestic measuring instruments, which often leads to probe burnout and no signal input to the instruments. To solve this problem, a mathematical model was established for transient wellbore temperature fields, which analyzed the field profiles at different parameters, and introduced the concept of “well depth at critical temperature” (WDCT). On this basis, in order to reduce the bottomhole circulating temperature and move the WDCT down to the bottom, the sensitivity of seven parameters were analyzed, including rheology, thermal property, displacement, and inlet temperature of drilling fluid, and the thermal property of the drill string. Then the physical parameters that could significantly influence the wellbore temperature field were obtained. It is found by analysis that WDCT could be divided into true, transitional, and equivalent categories according to the variation curves of wellbore temperature. The wellbore temperature field was sensitive to the thermal conductivity of the drill string, the specific heat capacity of drilling fluid, and the thermal conductivity of drilling fluid in a descending order, which corresponded to the equivalent WDCT. The results showed that changing the thermal properties of drilling fluid or reducing the thermal conductivity of the drill string could effectively lower the bottomhole circulating temperature in the Shunbei No.1 Area.