Abstract: To overcome the problems of high water cut and low recovery degree in the deep low-permeability heavy oil reservoirs of the Guan 128 block in Dagang Oilfield, a study on the oil recovery improving through viscosity reducer flooding and the optimization of development methods was carried out. By combining lab tests for microscopic seepage, core tests for relative permeability, and reservoir numerical simulation, the microscopic seepage law of viscosity reducer flooding were systematically analyzed, and high-efficiency development parameters were optimized based on the development characteristics of the target block. The results indicate that after injecting the viscosity reducer, the heavy oil undergoes in-situ emulsification in the porous media to form an O/W emulsion, thereby improving the microscopic oil displacement efficiency and expanding the sweep volume; during the later stage of displacement, the emulsion particle size gradually decreases; the influence of the Jamin effect is compromised, and the emulsion can migrate rapidly through pore throats, contributing to the residual oil mobilization. The parameter optimization results show that under the huff and puff mode of viscosity reducer , the optimal soaking time is 4 d; the optimal cyclic injection volume is 1 200 m³ (viscosity reducer solution), and the optimal viscosity reducer concentration is 2%; for the target block, adopting a single-slug injection method to inject a 0.5% viscosity reducer solution of 0.2 times pore volume as a slug can improve the recovery by about 5 percentage. The study demonstrates that viscosity reducer flooding can effectively improve the development performance of deep low-permeability heavy oil reservoirs through the synergistic effect of in-situ emulsification and flow regulation, which provides a theoretical basis and technical reference for the mechanism study and development plan design of viscosity reducer flooding in deep low-permeability heavy oil reservoirs.