The invention discloses a preparation method of polymer particle, which comprises that suspends monodisperse and swelling seed polymer particles with a first preset value of mass into a water solution containing stabilizer and surface activator, mixes polymerisable monomer with mass of second preset value and initiator with mass as third preset value, to be added into monomer solution at polymerisable temperature, to process reaction, while the reaction degree is controlled at 30-98%, adjusts temperature, adds reactant, swells, while the reactant contains a polymerisable monomer with mass of fourth preset value or a second crosslinkable monomer with mass of fifth preset value, and an initiator with mass of sixth preset value, heats to polymerisable temperature, to process polymerization. The inventive microballoon has better monodispersity, high yield, reaction reproduction support, reduced production cost, and controllable product shape and property, while the invention can prepare porous monodisperse microballoon product.