High efficiency viscosity-reducing agent which is used on THE thermal oil extraction technology, which is formed by the following raw materials with percentage by weight, firstly phenyl benzene, fused ring aromatic hydrocarbon or fused ring aromatic hydrocarbon derivative which does not contain chlorine element and sulfur element, secondly liquid petroleum product whose aromatic hydrocarbons content is no more than 5%, petroleum resin which is liquid state under the constant temperature or hydrocarbons natural aggregation which is particle state under the constant temperature, thirdly water-soluble anionic surface active agent, water-soluble non-ionic surface active agent, oil solubility anionic surface active agent or oil solubility non-ionic surface active agent, fourthly fusel oil, fifthly alcohol ether, sixthly oil solubility or water-soluble polymer which respectively has nonpolar portion which has eutectic crystal with ceresin wax molecular and polar portion which can make wax crystal system twisty on main chain and or branch chain, seventhly hyperoxide, eighthly carbamide, ammonium carbonate, ammonium acid carbonate or ammonium nitrite and ninthly ketone. The high efficiency viscosity-reducing agent has strong suitability, strong viscosity-reducing ability, safety and reliability, can largely reduce extract cost and increase the yield of oil pump, and increases the final recovery efficiency of oil reservoir.