To illuminate changes in the thermal stability of lignocellulosic biomass by homogeneous chemical modification in ionic liquids, sugarcane bagasse derivatives bearing carboxyl groups were prepared in ionic liquids. Fourier transform infrared (FT-IR) spectroscopy and solid-state nuclear magnetic resonance (NMR) confirmed the chemical structure of the derivatives. Sugarcane bagasse derivatives with degree of substituted OH as high as 9.93 mmol/g were achieved. The homogeneous esterification was demonstrated to be a more efficient approach than heterogeneous ones. Based on thermogravimetric analysis, the onset degradation temperature of sugarcane bagasse decreased dramatically to 185 °C, 160 °C and 140 °C, using succinic anhydride, maleic anhydride, and phthalic anhydride as reagent, respectively. A first-order degradation kinetic model was applied to obtain the degradation activation energies of sugarcane bagasse. The results showed that homogeneous chemical modification significantly decreased the thermal stability of sugarcane bagasse by reducing the onset degradation temperature and degradation activation energies.