Abstract: Heavy oil produced in the Henan Oilfield can be classified as extremely or super heavy oil, which can be developed economically and efficiently by using viscosity reducer with assistance of steam huff-and-puff. In earlier stage of development, formation contains less water, and water-soluble viscosity reducer can hardly reduce viscosity of water-in-oil emulsion. Accordingly, a temperature-resistant oil-soluble viscosity reducer was developed. Orthogonal test was conducted to determine formula of the GCS-YR oil-soluble viscosity reducer, namely, 3% vinyl acetate copolymer ZJ-3+2% fatty amine polyoxyethylene polyoxy propylene ether ZJ- 4+1% esterified modified polyether ZJ-5+20% tetralin ZZJ-6+74% solvent naphtha RJ-5. Through laboratory tests, optimal mass fraction of agent was determined to be 3%. Test results show that the formula system can endure temperatures up to 350 ℃ with capacities to reduce viscosity of low-water crude oil over 80%. In addition, the system displayed satisfactory compatibility with de-emulsifier of AR type for oil gathering and transmission in oilfields. Field tests of the viscosity reducer were performed in 6 wells in the Jinglou Oilfield with single-well productivity enhanced by 47 t and OGR enhanced by 0.03 on average. The GCS-YR oil-soluble viscosity reducer can be used for development of heavy oil after steam huff-and-puff to enhance recovery of heavy-oil reservoirs in the Henan Oilfield.