The effects of bulk density on the structure and combustion characteristics of rice husk (RH) char were studied under isothermal conditions. The char from each reaction stage was obtained through pyrolysis of RH/Al2O3. Thermogravimetric-Fourier transform infrared analysis was used to study the release of gaseous products during the fine particle pyrolysis of RH/Al2O3. The results showed that the bulk density of the particles seriously affected the release rate and product distribution of gaseous products during pyrolysis, especially in the release phase of volatiles. In comparison, biomass pellets had a faster pyrolysis reaction rate at low bulk densities, which resulted in lower contents of O and H in the char obtained at this stage, and the aromatic structure content slowly increased. In the carbonization stage, the low bulk density promoted the increase in surface area and pore volume expansion of char. The degree of aromatization in char was higher, but the carbon content decreased. The addition of Al2O3 limited the growth of aromatic crystallites, while the char obtained at high bulk density tended to be graphitized. The char reactivity at the two bulk densities first decreased and then slightly increased, but the char obtained at high bulk density had better combustion reactivity.