Provided are a preparation method of a thermoplastic resin composition and a thermoplastic resin composition prepared therefrom. The preparation method includes: feeding a first thermoplastic resin in a molten state into a first hopper and feeding an impact reinforcing agent including a predetermined content of moisture into a second hopper, followed by kneading and extruding processes, and removing the moisture in a first vent port and a second vent port, wherein the first hopper, the first vent port, the second hopper, a first kneading zone, and the second vent port are sequentially formed in a moving direction of an extruder. The thermoplastic resin composition prepared therefrom can have improved appearance property and improved color property by removing moisture of an impact reinforcing agent added to the thermoplastic resin composition.