The invention provides a preparation method and application of a pH sensitive type magnetic demulsifier with surface charges and hydrophilic-hydrophobic properties controlled. Based on magnetic Fe3O4 nanoparticles, through surface modification with silicon dioxide and a grafting copolymerization reaction, surface active monomers and pH sensitive monomer copolymer chains are introduced on the surfaces of the nanoparticles, and the surface grafting structure and the pH value are further adjusted to control the surface charges and hydrophilic-hydrophobic properties of the magnetic nanoparticles. The reduction of the pH value can enhance the hydrophobic property of the magnetic nanoparticles and reduce negative charge repulsive interaction between the demulsifier and oil droplets so that the demulsifier can be absorbed onto the surfaces of the emulsified oil droplets and quickly isolate the emulsified oil droplets under the action of an external magnetic field. Recycling of the magnetic nanoparticles can be achieved, and the demulsifier is be unlikely to be affected by pollution of negative-ion surface active materials in emulsified oil wastewater. According to the magnetic demulsifier, the preparation method is simple, the recycling process is convenient and efficient, the recycling performance is excellent, and therefore the magnetic demulsifier has very board application prospects in the fields of emulsified oil wastewater treatment and oil-water separation.