Potassium, due to its strong catalytic effects on biomass pyrolysis vapors, has great influence on the components of the obtained bio-oil. In this work, fast pyrolysis of acid washed and potassium salts impregnated feedstocks (camphor branch, corn cob and walnut shell) were carried out in a fluidized bed. The effects of potassium and the ash in biomass on yields and selectivities of bio-oil compounds were studied. The results showed that the walnut shells yielded the greatest amount of phenol and acids. Potassium promoted the conversion of large molecular compounds (such as levoglucosan) to furans. The existence of potassium reduced the yield of aldehydes and enhanced that of furfural. With the assistance of potassium, the selectivity of monophenols (phenol, methyl phenol, dimethyl phenol, etc.) decreased significantly to around half, while that of polyphenols (ethyl phenol, 2-allylphenol, 4-(2-propenyl)-phenol, 2-allyl-4-methylphenol, etc.) increased obviously. Finally, a possible reaction pathway indicating potassium influences on biomass pyrolysis was proposed according to the experimental results.