Microstructured optical fibers (MOFs) have attracted intensive research interest in fiber-based optofluidics owing to their ability to have high-efficient light-microfluid interactions over a long distance. However, there lacks an exquisite design guidance for the utilization of MOFs in subwavelength-scale optofluidics. Here we propose a tapered hollow-core MOF structure with both light and fluid confined inside the central hole and investigate its optofluidic guiding properties by varying the diameter using the full vector finite element method. The basic optical modal properties, the effective sensitivity, and the nonlinearity characteristics are studied. Our miniature optofluidic waveguide achieves a maximum fraction of power inside the core at 99.7%, an ultra-small effective mode area of 0.38 µm2, an ultra-low confinement loss, and a controllable group velocity dispersion. It can serve as a promising platform in the subwavelength-scale optical devices for optical sensing and nonlinear optics.