The heavy oil dispersed viscosity reducer has attracted much attention due to its unique functions and ability to reduce the processing link of crude oil. However，so far，the development of water-soluble dispersed viscosity reducer and the research on microscopic oil displacement mechanisms have been rare. Therefore，on the basis of testing the structure of heavy oil from Shengli Le’an Oilfield，the viscosity reduction mechanisms were studied through infrared spectrum analysis，transmission electron microscopy（TEM）and atomic force microscope scanning（AFMS）experiments on the heavy oil before and after adding the viscosity reducer；Using the CT and micro-etching models under high temperature and high pressure conditions，the micro-displacement mechanism of water-soluble dispersed viscosity reducer in the reservoir was studied.The viscosity reduction mechanisms are mainly as follows：the dispersing viscosity reducer penetrates and diffuses between the colloidal and asphaltene sheet molecules after it is combined with the heavy oil molecules；the combination of heterocyclic atoms in the viscosity reducer and colloids reduces the formation of complexes between family molecules and separates the aggregates；the original regular aggregates are transformed into sheet-like molecules with irregular distribution；the molecular structure becomes loose，the degree of order is decreased，and the entropy is increased，which reduce both the aggregation of heavy oil molecules and the intermolecular forces. Its microscopic oil displacement mechanisms in high temperature and high pressure reservoirs are mainly manifested as that，the dispersive intercalation has a stripping effect on the heavy oil adsorbed on the surface of rock particles and the efficiency is high at the conditions of low displacement rate and high concentration；the stripping effect is relatively weakened and there is a large amount of surrounding cluster-like residual oil at the conditions of high rate and low concentration.