Abstract: This paper deals mainly with calculation of heat transmission in the process of heating buried pipelines with hot water flowing in one direction. Since the initial start of buried pipelines results in three-dimensional unstable heat transmission, and at initial start the temperature change of the soil around a pipeline is affected by the temperature inside the pipeline and the ground surface temperature annual fluctuation, the differential equation for heat transmission and the boundary value maybe linearly concocted into two sets of equations for digital calculation to work out a calculation equation with electronic computer so as to know for sure that the hot water temperature on any location of a preheated pipeline at any time. The calculating method presented herein is applicable to engineering calculation as well.