Fast pyrolysis of bagasse catalyzed by activated carbon (AC) at low temperatures offered a new and promising way to produce the valuable 4-ethyl phenol (4-EP) compound in high selectivity. In this study, the technique of pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) was first applied to investigate several factors on the 4-EP production, including biomass type, fast pyrolysis temperature, AC-to-bagasse ratio, and catalytic pattern (in situ or ex situ catalysis). Moreover, fast pyrolysis experiments by using AC catalyst were conducted in a lab-scale setup to quantitatively determine the pyrolytic products. The experimental results indicated that, among these five herbaceous biomass materials, bagasse was the best material to produce 4-vinylphenol (4-VP) from the noncatalytic process and 4-EP from the catalytic fast pyrolysis process, respectively. 4-VP and its precursors could be catalytically hydrogenated into 4-EP with high selectivity under the catalysis of AC catalyst. Both catalytic pyrolysis temperature and AC-to-bagasse ratio affected the 4-EP selectivity significantly, whereas the catalytic pattern had minor effects on the 4-EP production. The highest yield of 4-EP from bagasse in Py-GC/MS experiments was 2.13 wt %, obtained at the pyrolysis temperature of 300 degrees C and AC-to-bagasse ratio of 4 from the in situ catalytic pattern. Moreover, lab-scale fast pyrolysis of bagasse catalyzed by AC catalyst obtained the maximal 4-EP yield of 2.49 wt %, with the selectivity of 10.71%.