The shale oil reservoir of the Lucaogou Formation in the Jimsar Sag currently faces bottlenecks such as rapid natural energy decline and high crude oil viscosity. While gas injection has emerged as the primary approach for shale oil exploitation, there is an urgent need for chemical viscosity reduction measures tailored to the reservoir conditions of the Jimsar Sag, given the high viscosity characteristics of its shale oil. This study investigates a compound flooding technology system combining a chemical agent system with CO? to provide an experimental basis for enhanced oil recovery in shale reservoirs. First, the physical properties of the Jimsar Sag shale oil reservoir were characterized through comprehensive analyses of porosity, permeability, pore-throat structure, and sensitivity evaluation, thereby delineating the reservoir features of the Jimsar shale oil play. Under formation temperature conditions, a ternary compound viscosity reduction system comprising anionic, nonionic, and amphoteric surfactants compatible with CO? was screened and its reservoir adaptability verified. On this basis, core huff-n-puff experiments were conducted to evaluate the improvement in oil recovery, and microscopic visualization experiments were performed to elucidate the compound flooding mechanism. The results indicate that the pores of the Jimsar shale reservoir are dominated by nanopores ranging from 10 to 100 nm, with relatively few micropores and a complex pore-throat network structure. The strong capillary resistance generated by the unimodal nanopore system is identified as the core factor restricting oil recovery. Optimized for formation conditions, the screened ternary compound system (2% sodium cetyl sulfate + 0.5% BS-12 + 1.2% NP-10) exhibits excellent performance, achieving a viscosity reduction rate of up to 96.24% and demonstrating effective adaptability to reservoir conditions. Under a pressure of 40 MPa, the combined CO?–chemical agent huff-n-puff process yields the most significant effect, with a cumulative oil recovery of 39.14%, representing an increase of 11.64% compared with pure CO? huff-n-puff.