Thermal treatments with water, diluted acid, and diluted alkali aqueous solution of poplar wood blocks were carried out in a Teflon-lined autoclave at three temperatures. The effects of different liquids and temperatures on wood surface color, cell wall microstructure, and chemical structures were investigated by the chromameter, scanning electron microscope (SEM), and Fourier transform infrared spectroscopy (FTIR). From the chromameter, it was observed that the lightness value decreased with temperature for all treatment conditions. The a* value increased with temperature in all liquid treatments. The b* value increased with temperature in hydrothermal and thermal with H2SO4 treatment but decreased with temperature in thermal with NaOH treatment. The total color difference (ΔE) was slightly changed in the hydrothermal treatment, but dramatically changed in the thermal with H2SO4 and NaOH aqueous treatments. SEM showed that the cell wall structure was damaged differently with different reagents and temperature. Middle lamella layers were always fractured in hydrothermal and NaOH treatments. However, both middle lamella and secondary cell wall were damaged after the H2SO4 treatment and intensified with temperature. These fractures usually parallel with the S2 layer microfibril angle (MFA) in the fiber cell wall. The FTIR analysis suggested that the chemical structure was obviously changed after the thermal with H2SO4 and NaOH treatments. And the missing or decreasing C=O absorption peak indicated hemicellulose is degraded and new compounds produced during thermal with H2SO4 and NaOH treatment. On the other hand, lignin was partly degraded in the H2SO4 treatment and guaiacyl nuclei was degraded before syringyl nuclei.