The invention discloses a preparation method of a tin dioxide doped titanium dioxide based thin film acetone gas sensor. The preparation method comprises the following steps of: firstly, preparing an SnCl4.5H2O solution with the concentration of 14.9%; dropping hydrochloric acid and ammonia water until the pH (Potential of Hydrogen) value of the solution is about 6 before the dropping of the ammonia water stops; agitating for 30min, ageing the mixture for 2 days at room temperature, and drying the aged mixture; preparing a butyl titanate solution with the concentration of 23.1% from absolute ethyl alcohol; mixing de-ionized water and acetic acid according to the volume ratio of 1:1; weighing prepared SnO2 powder and dissolving the powder into the mixed solution; adding SnO2 and TiO2 in a mol ratio of 1:(25-50); standing for three hours; drying the obtained mixture at 100 DEG C for 2 hours, and crushing the dried mixture in an agate mortar to obtain nano TiO2 powder doped with SnO2; mixing the powder and methylcellulose in a ratio of 1:1; coating the mixture on a ceramic pipe; annealing the pipe at 500 DEG C to obtain the sensor. A sensor is high in sensitivity to acetone, short in response recovering time and low in working temperature.