Cr2O3-TiO2 core-shell fibers (Cr-Ti CS-Fs) with different shell thicknesses were synthesized by a facile coaxial electrospinning technique. The gas-sensitive performance of Cr-Ti CS-Fs to acetone was enhanced by optimizing the TiO2 shell thickness. With the increase of the thickness of the TiO2 shell, the response of Cr-Ti CS-Fs to acetone showed a transition from p-type to n-type. Besides, the formation of Cr2O3-TiO2 heterojunction enabled the gas sensors to possess short response and recovery time to target gas. A possible mechanism of the enhancement of gas-sensitive performance of core-shell Fs was also illustrated.