In order to solve the problem of high development cost of heavy oil thermal recovery in Shengli oilfield，the viscosity reduction effect of a dispersive viscosity reducer L-A was investigated，the changes of molecular structure and adhesion force of heavy oil before and after viscosity reduction were analyzed. The micro-flooding mechanism was studied with microscopic visual oil displacement experimental model. The results showed that when the optimal concentration of dispersive viscosity reducer solution was 2% at formation temperature of 56 ℃，and the optimal volume ratio of viscosity reducer solution to heavy oil was 1∶6， the viscosity reduction rate reached up to 94.58%. When the viscosity reducer was mixed with heavy oil，the P—OH group and carboxylic acid —COOH in the viscosity reducer intercalated into the heavy oil molecules，as a result，the complex sheet molecular structure of gum and asphaltene was broken，and the space network structure of colloidal asphaltene was destroyed. The molecular structure became loose，the association effect was weakened，the adhesion force decreased，and the internal friction between molecules declined，which led to the viscosity of crude oil decreased obviously. On the microscopic level，the dispersive viscosity reducer L-A could disperse and separate the remaining oil from the blind end of the throat，parallel throat and small hole throat of the low permeability channel，demonstrating the ability of oil displacement efficiency and profile control.