We propose a novel mode-selection method in all-fiber laser cavity based on composite optical microsphere cavity Whispering-Gallery-Mode（WGM）resonance spectra and simulate and analyze the optical field distribution of the microsphere cavities constructed with multilayer media and parallel structures by using the Finite Difference Time Domain（FDTD）algorithm.On the basis of the optimal conditions for coupling the microsphere cavities with tapered fibers

we calculate and obtain their transmission spectra.By optimizing the parameters of microsphere cavities and improve the coupling conditions

we compress the WGM resonance spectra bandwidth and enhance the side-mode suppression ratio.The acquired narrow-band transmission spectra are of major significance to all-fiber laser mode selection.