Mode and refractive index sensing properties of silica capillary microbubble were studied numerically using a finite element method. Mode characteristics such as quality factor, effective refractive index and percentage of light intensities inside the core were determined for different bubble diameters and shell thicknesses, and the potential application of microbubble in the high sensitivity and high resolution refractive index sensing was discussed. The results show that, optimized shell thicknesses sizes for the best sensitivity and resolution can be obtained, which is about 1 μm for microbubble with a diameter of 350 μm. Around this thickness, the resolution does not vary too much with bubble size, the sensitivity of the second order mode with a high quality-factor is higher than that of the first order mode. Therefore, in a practical sensing application, the second order mode is recommended since it can reduce the level of control needed on the shell thickness during device fabrication. It has a certain theoretical reference value to guide the experimental production process.