The invention discloses a method for exploiting thickened oil by steam solvent mixed injection. The method comprises the following steps of: starting to inject a solvent at the early stage of development, firstly performing high-pressure production, and accelerating cavity expansion and oil drainage; performing pressure reduction production at the middle and later stages of development, improving the utilization efficiency of heat energy, and meanwhile strengthening solvent recovery; heating the solvent, performing phase change, enabling the solvent to enter an oil reservoir in a gas phase form, reaching the front edge under the convection action, condensing, mixing with crude oil, and reducing the viscosity of the crude oil; performing steam solvent mixed injection, reducing the partial pressure of steam, improving the dryness, and reducing the heat loss of a pipeline along the way; performing flash evaporation and heat absorption on hot water and the solvent inside a steam cavity in the pressure reduction process, thereby recovering part of heat energy inside the steam cavity, and transporting to the front edge of oil drainage; and reducing the solubility of the solvent in oil water after pressure reduction, reducing the adsorption quantity on the surface of stratum minerals, and strengthening the recovery of the solvent.