The invention discloses a demonomerisation method of a polyacrylonitrile spinning solution. The method comprises the following steps: step 1: uniformly distributing an acrylonitrile polymerization solution onto the end surface of a packing layer from a position above packing on the top of a packing tower by virtue of a material distributor; step 2: meanwhile, at a negative pressure state, introducing solvent vapor below packing on the bottom of the tower, performing the vapor-liquid exchange on the solvent vapor flowing by the packing layer from bottom to top and the acrylonitrile polymerization solution flowing downwards from top, discharging residual volatile monomer in the acrylonitrile polymerization solution by virtue of a vacuumizing system, and then completing the demonomerisation. The packing tower is used for demonomerisation, so that materials are enabled to form a large-enough specific surface area, sufficient demonomerisation time of material liquid in the tower is guaranteed, and the demonomerisation efficiency is greatly increased; by means of the way for introducing the solvent vapor from the tower bottom, the differential pressure of volatile components can be reduced, further devolatilization is facilitated, the mass transfer of volatile components on the surface of polymerization solution is improved, and the volatile components can be brought away by a great amount of solvent vapor, so that the demonomerisation effect of a product is greatly improved.