The sensitivity enhancement effect of microtube coated with a dielectric layer on the inner surface is presented. The simulations of mode-field distribution and senstivity are carried out for thin wall microtubes and thick wall microtubes with an established four-layer ring structure model. The results show that optical field energy in the core of the microtube can be increased when a dielectric layer with high refractive index is coated on the inner wall surface of the microtube, thus the sensitivity is enhanced. The sensitivity of the 1st-order radial mode of the thin wall microtube and the high-order radial mode of the thick wall microtube are enhanced by 65 and 302 times, respectively. The refractive-index sensing sensitivity of the high-order radial mode will experience multiple peaks accompanying with the thickness change of the dielectric layer, which is analyzed from the view of geometrical optics.