The alkali and alkaline earth metals in the biomass are one of the major challenges that restrict the biomass/low-rank-coal entrained-flow gasification in the way of ash slagging and corrosion. To clarify the corrosive behavior from bio-slag on the refractory material of the gasifiers, this study investigates the blending of biomass with coal for high-temperature corrosion on an alumina refractory in a vertical furnace under the reducing environment. The ash samples from a typical wheat straw (WS) and a Shenhua bituminous coal (SH), as well as their blends in three different mass ratios at 10 wt % WS, 30 wt % WS, and 50 wt % WS, were selected for high-temperature corrosion tests at 1350 °C for 2 h holding time. The obtained slag, especially the cross section, was examined using optical microscopy, scanning electron microscopy (SEM) coupled with energy-dispersive spectroscopy (EDS), and Raman spectral analysis. Additionally, the thermodynamic calculation was conducted using Factsage. The results show that the addition of WS into SH lowers the ash fusion temperature. The WS30 blends have the lowest ash fusion temperature with a fully molten phase at 1130 °C. The addition of WS promotes the slag fluidity, while adding 30% of WS into SH results in the largest spreading area. Interestingly, the slag with a higher fluidity is more corrosive, indicated by a larger filtrated area and depth in the cross section. Elemental mapping results show that the ash-forming elements are evenly distributed in the slag for fully melted samples. However, the Mg species precipitated near the slag-refractory interface and react with alumina to form a spinel phase that prevents further penetration.