The invention relates to a preparation method of a water-soluble anion polymer-type thickened oil viscosity reducer. The thickened oil viscosity reducer comprises a component A and a component B; thecomponent A is an acrylamide copolymer containing acryloyl morpholine blocks; the component B is an acrylamide copolymer containing active end groups. A thickened oil viscosity reducer solution is mixed with thickened oil; it is proved through viscosity reduction tests that when the dosage reaches 0.2 mg/mL, the viscosity reducer can achieve high-temperature stirred emulsification and viscosity reduction in pure water and salt water, the viscosity reduction rate is greater than 93%, and stirred demulsification is stopped at room temperature. The thickened oil emulsification viscosity reducer is obvious in viscosity reduction effect and high in environmental adaptability, the emulsification and viscosity reduction behaviors of the thickened oil viscosity reducer have temperature and shearing responsiveness, and convenience is provided for application and post-treatment in oil fields.