The invention relates to the energy-saving technology of distillation columns, in particular to a tower of an external thermally-coupled composite distillation column and a control process thereof, which are applied to the categories of integration and design of chemical processes, effective utilization of energy, environmental protection, and the like. Aiming at the problem that the tower of theexternal thermally-coupled composite distillation column is difficult to realize in the chemical process, the invention provides a novel tower of the external thermally-coupled composite distillationcolumn, and utilizes the thermal coupling action between a rectifying section of a high-pressure distillation column and a stripping section of a low-pressure distillation column similar to three external heat exchangers to skillfully avoid the problem that an external thermal coupling structure is difficult to realize and ensure that the tower of the external thermally-coupled composite distillation column has high thermodynamic efficiency.