Abstract: The traditional viscosity reducer has low stirring strength in the process of auxiliary heavy oil lifting, and can not be fully mixed with the produced liquid. Therefore, there will be a problem that the viscosity reduction effect of indoor evaluation is good, but the effect of on-site application is not good. To solve this problem, a viscosity reducer was developed, which can achieve high viscosity reduction under low stirring strength. A polyether viscosity reducer for heavy oil was synthesized with styrene, maleic anhydride and nonylphenol polyoxyethylene ether as main raw materials in laboratory, and its viscosity reduction performance was investigated, and the viscosity reduction mechanism was discussed. The results show that the agent can achieve a viscosity reduction rate of 95.6% under the condition of stirring at 50 r/min for 1min, and can still maintain a viscosity reduction rate of 87.2% after 24h, which showing strong viscosity reduction ability and viscosity reduction under low stirring intensity stability and viscosity reduction under low stirring intensity stability. Scanning electron microscope (SEM) was used to observe the morphology of colloidal asphaltene before and after the addition of the agent. It was found that the addition of the agent dispersed the original agglomerated structure of colloidal asphaltene, which proved that the agent reduced the viscosity through dispersion.Observing the morphology of the colloidal asphaltene before and after the addition of the drug by scanning electron microscopy, it is found that the addition of the agent breaks up the original agglomerated structure of the colloid and asphaltene, which proves that the agent can reduce viscosity through dispersion.