Wood-based activated carbon microspheres (WACMs) were successfully prepared from Chinese fir. However, the weak thermal stability of the precursors resulted in melting at high activation temperatures, which led to a lower yield and poor adsorption properties. To promote the thermal stability of the precursors in the process without chemicals, this paper focused on the effects of the pre-oxidation before activation on the thermal stability and chemical structure of the precursors and the pore structure of the activated samples. The results showed that during the pre-oxidation process, the O/C ratio increased noticeably and the intensity of oxygen-containing functional groups, such as carbonyl, carboxyl, and ester groups, were enhanced, which contributed to the improvement of the thermal stability. Moreover, the etching effect of oxygen on the structure of the precursors was verified when the porous structure was enriched by activation. The activated sample pre-treated at pre-oxidation temperature 250 °C had the maximum micropore specific surface area and pore volume. As the temperature was raised to 280 °C, the mesopore specific surface area and pore volume increased, which indicated that the optimization of the pre-oxidation parameters contributed to the formation of the mesopore structure.