Mode splitting caused by the coupling between whispering gallery mode microcavities will lead to the increase of Q factor of the device, and hence the performance of the device can be enhanced. As a new type of microcavity coupling form, an in-fiber coupled microsphere resonator improves the integration and stability of fiber devices. In this paper, an optical fiber sensor based on the fiber coupled double microsphere resonators is fabricated and investigated, which consists of the single mode fiber, silica capillary, and two barium titanate microsphere resonators. The whispering gallery modes of two microsphere resonators are coupled to each other, where the induced mode splitting increases the Q factor from 8×10 ³ to 2.4×10 ⁴. As mode splitting is beneficial to improve the sensing ability of small changes, the device is demonstrated to own a good temperature response stability in temperature sensing experiments, with the temperature sensing sensitivity of 11.7 pm /℃, and the detection limit is as low as 0.03 ℃.