The invention discloses a permeability-reducing agent which comprises a mixed monomer, a cross-linking agent and an initiating agent. The mixed monomer includes acrylic amide and acrylic acid in the mass ratio of 1:1-3:1; the quality of the cross-linking agent is 0.1%-0.3% of that of the mixed monomer; the initiating agent includes ammonium sulfate and sodium sulfite, the quality of ammonium sulfate is 0.5%-2% of that of the mixed monomer, and the quality of sodium sulfite is 0.5%-2% of that of the mixed monomer. The invention further discloses a preparation method and application of the permeability-reducing agent. When the permeability-reducing agent is applied to salt pan permeability-reducing project, only a mixed-monomer water solution and an initiating-agent water solution are needed to be evenly sprayed on a salt pan directly, and a permeability-reducing layer can be instantly formed in seams and the surface of salt pan soil; the permeability-reducing agent has the advantages of simpleness in process, easy operate, fitness for large-scale operation on salt pan areas, low cost, good permeability-reducing effect, low requirement on operation conditions and the like.