This invention relates to a method for distilling an isomer mixture of isocyanate. The method comprises: adding an isomer mixture of diphenylmethane diisocyanate into a separator, and distilling to obtain at least one product mixture, which comprises: 2,4'-MDI 50-60 wt.%, 4,4'-MDI 40-50 wt.%, 2,2'-MDI less than 1.5 wt.%, 2,4'-MDI less than 1 wt.%, and 4,4'-MDI greater than 99 wt.%. Compared with present distillation methods, the method has such advantages as saved distillation steps, reduced distillation column number, saved energy input, reduced isocyanate loss, and high 4,4'-MDI purity in the product mixture.