Binderless poplar/bismuth oxide wood alloy is prepared using the warm-press forming technology. The effect of the forming temperatures on color changes and mechanical properties of the poplar/bismuth oxide wood alloy is studied. The results show that the surface color of the specimen gradually darkened as the forming temperature increased. There is the most obvious change from 140 to 160 °C. The CIE lightness color coordinate L* and chroma coordinate b* decrease with the increase of the forming temperature, while chroma coordinate a* decreases initially, but later increases with treatment severity. The static bending strength (MOR), the elastic modulus (MOE) and the surface hardness (HV) increase first and then decrease with the increase of the forming temperature. The X-ray diffraction (XRD) analysis shows that the wood underwent carbonization at 180 °C, resulting in a decrease in the density and mechanical properties of poplar/bismuth oxide wood alloy, and a deepening of the surface color. The Fourier transform infrared spectroscopy (FTIR) analysis reveals that the pyrolysis of cellulose and hemicellulose, as well as the pyrolysis and condensation of lignin led to the color of poplar/bismuth oxide wood alloy deepening. The hydroxyl groups between the cellulose molecular chains are reduced and hydrogen bonds are formed at 140 °C, which improve the mechanical properties of poplar/bismuth oxide wood alloy. However, the massive degradation of hemicellulose weakens binding strength with cellulose and lignin at 160 °C above. It greatly reduces the mechanical properties of specimen.