The invention discloses a preparation method of a macromolecule organic membrane, in particular to a preparation method of a polystyrolsulfon acid salt/polyethyleneimine crosslinking nanofiltration membrane. The preparation method comprises the following steps of: taking polystyrolsulfon acid salt and polyethyleneimine as solute, dissolving in water, compounding a layer of a crosslinking ultrathin function layer on a supporting membrane by means of spray gel, drying and gelling polymer with acid till that a composite membrane is formed. The preparation method has the advantages that the membrane compounding technology is novel, the technology condition is easily controlled, the nanofiltration membrane has good hydrophily and higher intensity, the crosslinking polymer is provided with a plurality of various charged functional groups to be capable of effectively adjusting the selection, the hydrophily, the hydrophobicity and the compatibility of the obtained membrane, and the composite membrane has higher interception to inorganic salt. The membrane prepared by the preparation method can be widely applied to various separation industries, in particular to the separation of water solution.