Microstructured optical fibers (MOFs) have been widely utilized for optofluidic waveguides, in which the micro-holes along the MOFs act as microfluidic channels for various applications such as supercontiuum sources and aqueous solution sensors. The air hole diameter of MOFs is generally several microns to tens of microns, which is much smaller compared with traditional fluid-core fiber waveguide. However, for specific applications such as ultra-trace analysis and low-threshold fluidic lasers, this channel dimension is still relatively large. The slot waveguide could confine light in a subwavelength-scale channel, however, the short propagation length and high loss of the waveguide may limit its applications. Here we propose an approach to achieve low-loss subwavelength-scale optofluidic waveguide in MOFs. As shown in Fig. 1(a), the hollow-core of the MOF is tapered down to a subwavelength-scale, and then the MOF is selectively infiltrated with aqueous solution. So light could be confined in an aqueous core surrounded by a tapered air-silica cladding. As the refractive index of the cladding is close to that of air, the index contrast between the fluidic core and the cladding is high enough to provide an efficient index-guiding in a subwavelength-scale core. The mode pattern of a fundamental mode in a tapered aqueous core MOF is calculated when the core diameter is 600nm (Fig. 1(b)). As shown in Fig. 1(c), we compare our tapered air-silica cladding with Teflon cladding. At the wavelength of 633 nm, the light with the fraction of power over 65% could be guided in the aqueous core with the size of 600 nm, which is 4 times higher than that in a core surrounded by a Teflon cladding, which has an overlap fraction of only 15.7%. So the method we propose has obvious advantages over the traditional method using Teflon material. It provides a novel platform for low-loss subwavelength-scale fluid-core guiding with the long interaction length, and therefore have various potential applications.