The invention discloses temperature-controlled variable-viscosity acid and a preparation method thereof. The variable-viscosity acid is prepared from the following components in percentage by mass: 12%-20% of hydrochloric acid, 0.6%-1.5% of a temperature-controlled variable-viscosity acid thickening agent, 0.3%-1.0% of a temperature-controlled variable-viscosity acid activator, 1%-2% of a corrosion inhibitor, 1%-2% of a ferric ion stabilizer, 1%-2% of a demulsifier, 1%-2% of a cleanup additive and the balance of water, wherein the corrosion inhibitor, the ferric ion stabilizer, the demulsifier and the cleanup additive with good compatibility are selected. The preparation method comprises steps as follows: preparation of the variable-viscosity acid thickening agent, preparation of the variable-viscosity acid activator and preparation of the temperature-controlled variable-viscosity acid. A fracturing fluid prepared from the variable-viscosity acid has the characteristics of low low-temperature viscosity, high high-temperature viscosity, realizes temperature-controlled variable viscosity, is good for increasing action distance of an acid liquid and realizing deep acidification, improves the pumpability and reduces the pipeline friction; besides, the variable-viscosity acid also has good anti-temperature shear resistance, corrosion inhibition and retarding property, and can completely break gel.