The low flow characteristics of heavy oil have brought many challenges to its exploitation. Looking for a cost-effective crude oil flow improver to reduce the viscosity of heavy oil is currently the most important challenge. Small-molecule crude oil flow improvers meet the above requirements and become an ideal target for oilfield heavy oil production. In this article, three kinds of barium salts of alkylbenzene sulfonates with different lengths of alkyl chains were synthesized from alkylbenzene sulfonic acid and barium hydroxide. Among them, the viscosity reduction effect of barium dodecylbenzenesulfonate (BaDBS) is best as the dosage is 900 mg/L, the viscosity reduction rate is 89.0%, and the pour point is reduced by 5°C. Optical microscopy revealed the eutectic effect of crude oil flow improver and saturated hydrocarbon in heavy oil. FTIR and DSC were used to research the mechanism of small-molecule crude oil flow improver to reduce the viscosity of heavy oil.