Abstract: The slow and fast co-pyrolysis characteristics of cellulose and oxalic acid were investigated by thermogravimetric-Fourier transform infrared spectroscopy and horizontal fixed-bed pyrolysis setup. The weight loss curve of slow co-pyrolysis showed oxalic acid decomposition and cellulose decomposition stages. As the decomposition of oxalic acid and cellulose was not synchronous, oxalic acid affected the decomposition of cellulose mainly through the volatiles formed by its decomposition, which was not obvious. Differently, in fast co-pyrolysis, oxalic acid and cellulose were simultaneously pyrolyzed, and sufficient interaction could occur between raw materials and volatile components. Therefore, oxalic acid had a significant impact on the pyrolysis products of cellulose. Compared with the fast pyrolysis of cellulose, the contents of levoglucosan and levoglucosenone decreased, while the content of 1,4∶3,6-dianhydro-α-D-glucopyranose increased significantly in the bio-oil during fast co-pyrolysis process. The volume fraction of CO in pyrolysis gas decreased, whereas that of CO₂ increased. In addition, the decomposition of cellulose was more thorough, and more aromatic structures were formed in biochar.