Preheating is a key step for the start-up of new submarine hot oil pipelines, which is characterized by the unsteady hydraulic and thermal coupling, and affected by many factors, including the fluid flow rate and temperature of preheating medium, the properties of subsea soil, the temperature of seawater, etc. To investigate the quantitative effects on the preheating results caused by the above influential factors, simulation research is performed in this paper with a numerical methodology named DMOC-FEM established before by taking a new Chinese submarine crude oil pipeline as example. The factors studied include the flow rate and inlet temperature of preheating medium, riser, physical properties of subsea soil, temperature of seawater, etc. The results show that the preheating medium mainly warms submarine pipe walls rather than the surrounding subsea soil. Among the influencing factors, the fluid flow rate and temperature of preheating medium have the most notable and direct effect on the distribution and change of fluid temperature in the subsea pipe. That caused by other factors happens to be much lighter in comparison. Due to the excellent insulation effect of the pipe-in-pipe structure, the temperature drop happens on the subsea pipe risers can be ignored. Furthermore, among the properties of subsea soil, thermal conductivity has the greatest influence on the outlet temperature during the preheating process.. [ABSTRACT FROM AUTHOR]