The invention provides an underground upgrading viscosity-reducing oil production method for thick oil. The underground upgrading viscosity-reducing oil production method for the thick oil comprises the steps that in the process of steam injection production of the thick oil, amphiphilic catalysts and additives are injected into a stratum along with steam, an oil layer is heated to enable the temperature to range from 160 DEG C to 240 DEG C, and soaking operation is conducted for 1-5 days, so that an aquathermolysis reaction between asphaltene and colloid in heavy components of the thick oil is facilitated, wherein the total mass of the amphiphilic catalysts and the additives is 0.15-2.0wt% the mass of the thick oil, the amphiphilic catalysts are prepared through a reaction between nickel salt and aromatic ring acid, and the additives are hydrogen donors and/or dispersing agents. According to the invention, the amphiphilic catalysts are hydrophilic and oleophilic, dissolve and disperse in the steam, and then disperse in the thick oil of the stratum along with the steam. Under the existing steam injection condition, the amphiphilic catalysts are added to the steam, and site operation can be conducted. The underground upgrading viscosity-reducing oil production method for the thick oil is easy to operate, and safe and reliable, and has the broad application prospect and the high promotion value, the production cycle is remarkably prolonged, and the good effect of enhanced production of the oil can be achieved.