In order to realize the whispering gallery microcavity with compact structure and stable performance, this paper proposes an in-fiber parallel dual microdisk structure, in which the whispering gallery microcavity and the optical waveguide that excites the whispering gallery mode are both in the fiber core. Finite time domain difference method is used to study the variations of the symmetric mode and antisymmetric mode in the mode splitting with the coupling distance between two microdisks. The research results show that there is a competitive relationship between the symmetric mode and the antisymmetric mode. As the coupling distance increases, the transmittance of the antisymmetric mode first increases and then decreases, and the resonance wavelength is red shifted; the symmetric mode is exactly the opposite. The influence of manufacturing process accuracy on mode splitting is analyzed by calculation. The research results can be used to design high-sensitivity fiber-optic temperature sensors in fiber micro-disks and other fields.