Abstract: The traditional viscosity reducer has low stirring strength in the process of auxiliary heavy oil lifting, and it fails to be fully mixed with the produced liquid, leading to a poor on-site application effect. To solve this problem, a polyether viscosity reducer for heavy oil (MSN) was synthesized with styrene, maleic anhydride, and nonylphenol polyoxyethylene ether as main raw materials, and its viscosity reduction performance was investigated by laboratory tests. The viscosity reduction mechanism was discussed. The results show that MSN can achieve a viscosity reduction rate of 95.6% under the condition of stirring at 50 r/min for 1 min and can still maintain a viscosity reduction rate of 87.2% after 24 h. It also shows strong viscosity reduction ability and viscosity reduction stability under low stirring intensity. Scanning electron microscope (SEM) is used to observe the morphology of colloid and asphaltene before and after the addition of MSN and results show that the addition of MSN significantly disperses the original agglomerated structure of colloid and asphaltene. The analysis finds that MSN reduces the interaction between colloid and asphaltene molecules in heavy oil through dispersion and prevents them from forming a colloidal stable structure, so as to achieve the purpose of viscosity reduction in heavy oil. The successful development of viscosity reducer MSN provides technical support for the efficient development of heavy oil.