The invention provides a lubricating oil viscosity index improver and a preparation method thereof; the lubricating oil viscosity index improver has stable chemical performance, oxidation resistance, shear resistance and good compatibility with lubricating oil; in addition to good thickening ability to the lubricating oil, the lubricating oil viscosity index improver further has excellent anti-shear stability and viscosity-temperature performance. The viscosity index of the lubricating oil after modification is greater than 150, and low-temperature kinematic viscosity is less than 1600mpa.s. The lubricating oil viscosity index improver can not only be used for preparing multi-level lubricating oil, but also be used for improving the viscosity index of base oil of the lubricating oil with the low viscosity index, thereby having broad application prospects.