In order to systematically study the mechanism of enhanced oil recovery（EOR）of viscosity reducer flooding，which is a new development method，a single-tube sand-packed model，a three-dimensional sand-packed model，and a microscopic glass etching model were used to test the oil displacement efficiency and the sweep efficiency of the viscosity reducer flooding，and analyze its causes. The experimental results show that the viscosity reducer flooding enhances oil recovery by increasing the oil displacement efficiency and the sweep efficiency. Compared with water flooding，the viscosity reducer flooding improves the oil displacement efficiency by 13%，and the mechanisms are as follows：①the crude oil is dispersed and emulsified to form small oil-in-water droplets，which is conducive to reducing the apparent viscosity of crude oil through a narrow throat；②the interfacial tension is reduced，the number of capillaries is increased，and the residual oil saturation is reduced. Simultaneously，the viscosity reducer flooding increases the sweep efficiency from 18.8% to 39.9%.The mechanisms are as follows：①emulsion profile control and flooding. The dispersed and emulsified crude oil enters the water channeling channel，which causes the water percolation area to decrease and the resistance to increase，and the subsequent injected liquid enters the previously unswept area；②Jamin effect. The small viscosity-reduced and emulsified oil drops aggregate to form large oil drops blocked at the throat，and the surrounding displacement fluid turns. This study clarifies the EOR mechanism of viscosity reducer flooding，and lays the foundation for the subsequent studies of the development technical limit and field application.