The uniformity principle of temperature difference field is a phenomenological principle，which has not been theoretically provedFor onedimensional two-and three-stream heat exchangers，the extremum principle of entransy dissipation was used to optimize the heat transfer process by variational calculusIt was indicated that the temperature difference field between the hot and cold fluids should be completely uniform if the entransy dissipation reached a minimum for a given heat duty，or if the heat duty reached a maximum for a given entransy dissipationSo，the uniformity principle of temperature difference field of heat exchangers was primarily proved.