A tapered-optical fiber tip coupler for the excitation of capillary whispering gallery modes (WGMs) is proposed. The phase matching theory and spectral features are analyzed. The light transmission characteristics and cross-sectional mode fields of the coupler are simulated. The impact of fabrication tolerance on excited WGM is studied, and the structure is fabricated based on the simulation results. It has been experimentally verified that this structure can excite the WGM resonance of capillaries of various sizes, with the highest quality factor (Q factor) of 3.6×103 and refractive index sensitivity of 22.28 nm/RIU. The resonant coupler is not fragile, large fabrication tolerance, and can excite WGM in capillaries of different sizes, which can provide a new means of detection in the field of capillary biochemical sensing.