The invention discloses magnetic porous cationic polymer adsorbent and a preparation method thereof. The preparation method comprises the following steps of ultrasonically and evenly emulsifying ferrous chloride tetrahydrate, ferric chloride hexahydrate, polyethyleneimine and chitosan; generating ferroferric oxide nanometer particles in an in-situ nucleation mode after heating reaction, wherein the magnetic nanometer particles are included by molecular chains of the chitosan and the polyethyleneimine to form a ternary network interpenetrating composite during nucleation growth; then performing cross-linking graft on the polyethyleneimine and the composite through cross-linking reaction, so as to obtain the polyethyleneimine-chitosan magnetic porous cationic polymer adsorbent. The adsorbent has a good adsorbing effect on congo red; when the batch charge of the adsorbent is increased to 25mg, the removal rate of 100mg/L (50ml) congo red can be 95% or more; the adsorbent has a simple and quick magnetic separation function, is beneficial to simplifying a general centrifugation and settlement process which consumes time and energy in a water treatment process, and has good application prospect in the field of sewage treatment.