The invention discloses a porous polymer nanoparticle and a preparation method thereof. The method is characterized by adding 10-200 parts of polymers, 0-200 parts of additives and 300-1000 parts of solvents by weight into a dissolution kettle with a stirrer and a thermometer, stirring to dissolve the mixture at 30-100 DEG C and obtaining polymer solution through defoaming and aging; and placing the polymer solution into an electrostatic field under 5-25KV to prepare the porous nanoparticle through electrostatic spraying. Smooth split of the nanoparticle is realized by a frozen breaking method, thus achieving the aims of minute observation, research and control of the inside structure of the nanoparticle. Electron microscope scanning test result shows that the diameter of the nanoparticleis distributed from hundreds of nanometers to thousands of nanometers, and the nanoparticle matrices are all of porous structure. The porous nanoparticles in the shapes of balls, red blood cells and round lanterns can be prepared by changing the technological conditions. The nanoparticle has the advantages of small diameter, large specific surface area and controllable surface and inside structure and is used in such fields as drug release, catalyst carriers, chemical sensors, biomolecule analysis, gas chromatography, ion exchange and polymerization carriers.