The viscosity reduction cold recovery of heavy oil is a crucial method for heavy oil extraction. Utilizing humic acid as the starting material, a novel humic acid-based viscosity reducer for heavy oil was designed and synthesized through sulfonation and condensation polymerization. This viscosity reducer exhibits excellent water solubility and a relatively high molecular weight. The process optimization of the synthesized viscosity reducer was achieved by adjusting the dosages of sodium sulfite and formaldehyde, with the optimal dosages determined as 20% and 40%, respectively. Concurrently, thermogravimetric analysis was employed to characterize the product. The viscosity of heavy oil was analysed based on varying amounts of the viscosity reducer added. At an optimal dosage of 0.38%, the heavy oil exhibited the lowest apparent viscosity of 598 mPa·s, demonstrating a significant viscosity reduction effect.