The effect of reaction conditions, temperature, heating rate, and reaction atmosphere on the formation of polycyclic aromatic hydrocarbons (PAHs) from lignin was studied in a fixed-bed reactor. When the temperature was increased from 500 to 900 °C, most PAHs increased with temperature, except 1-methynaphthalene and 2-methynaphthalene, which decreased slightly when the temperature was increased from 800 to 900°C. With the increase of the temperature, the percentage of 2-ring PAHs decreased and the percentage of 3- and 4-ring PAHs increased. The increase in the total PAH with the temperature could be fitted by a quadratic function. The PAH generation from slow pyrolysis of lignin was much lower than that from fast pyrolysis. In comparison of the PAH generation in different reaction atmospheres, experiments in N2 produced the most PAHs, followed by the reaction in air and CO2. During the pyrolysis/gasification of lignin, it is suggested that there were two kinds of secondary reactions, dehydroxylation and demethoxylation, and they might occur at the same time. Then, PAHs could be formed from secondary reactions of derivatives of benzene, which increased with the increase of the temperature. Slow pyrolysis generated less PAHs because of the limitation of secondary reactions. With the addition of air or CO2, derivatives of benzene and phenol could be oxidized; thus, less PAHs were generated.