The invention provides a hydroxypropyl chitosan microsphere, and a preparation method and application thereof in reducing the dielectric loss of mineral oil, and belongs to the field of chitosan modification. The preparation method comprises the following steps: mixing chitosan, isopropanol, and a first sodium hydroxide solution; adding tetramethyl ammonium hydroxide, dropwise adding epoxy propaneto carry out reactions; cooling, adjusting the pH to 7 by using hydrochloric acid; dispersing the reaction product in acetone or water, stirring, carrying out suction filtration, washing the reactionproduct by anhydrous ethanol, drying the reaction product in vacuum to obtain hydroxypropyl chitosan; preparing a hydroxypropyl chitosan solution, spraying the solution on sodium polyphosphate to carry out cross-linking reactions, washing the reaction product by water, filtering, adding the reaction product into a sodium hydroxide solution, stirring, adding epoxy chloropropane to carry out reactions to obtain microspheres; washing the microspheres by water, then drip-washing the microspheres by organic solvents with volume percentages of 10-20%, 30-40%, 70-80%, and 90-95% in sequence, and finally performing freeze-drying. The prepared modified chitosan can reduce the dielectric loss of mineral oil.