The high temperature in the horizontal section and bottom of a horizontal well has a significant impact on the performance of mud and the safety of drilling equipment. The high bottomhole temperature is lowered often by reducing the inlet temperature. To explore how the inlet temperature impacts the bottomhole temperature during horizontal well drilling, a computational model for transient temperature during horizontal well drilling was established in this paper. Finite difference approach was utilized for the model discretization, while the relaxation iteration technique was adopted for the model resolution. The influence of inlet temperature on bottomhole temperature was analyzed during alterations of inlet flowrate and horizontal section length. On the basis of the obtained results, the impact of inlet temperature on bottomhole temperature decreases as the horizontal section length increases. On the premise that the horizontal section is long enough, lowering the inlet temperature exerts little impact on the bottomhole temperature fluctuation. As the inlet flowrate declines, the influence of inlet temperature on bottomhole temperature decreases. In addition, with the inlet flowrate being small enough, lowering inlet temperature rarely impacts the bottomhole temperature fluctuation.