The washing pretreatment and torrefaction pretreatment of corn stalk were performed in this study. The effects of both separate and combined pretreatments on the pyrolysis were studied using TG-FTIR and a fixed bed reactor. Washing pretreatment had little impact on the physicochemical properties of corn stalk, contributing mainly to the removal of some ash and metallic species. Torrefaction pretreatment, on the contrary, decreased the oxygen content but increased the ash content. TG-FTIR analysis showed that TG/DTG curves of corn stalk (CS), washed corn stalk (W-CS), torrefied corn stalk (T-CS), and torrefied-washed corn stalk (TW-CS), in turn, showed a right shift trend, and the initial decomposition temperatures increased obviously. Among the samples, W-CS had the lowest values of 27.58 and 29.76 wt % for the final residue mass in the TG curve and the biochar yield, respectively, and the highest bio-oil yield of 48.32 wt %, indicating that the removal of metallic species facilitated the pyrolysis of corn stalk and reduced the secondary cracking of pyrolysis volatiles. Moreover, torrefaction pretreatment greatly promoted the generation of combustible gases and phenols of bio-oil, whereas it remarkably reduced the acids of bio-oil. The combination of water washing and torrefaction preserved the advantages of each method on its own, illustrating this combination was a promising pretreatment for improving pyrolysis products.