The shape selectivity of molecular sieve catalyst is caused by the difference between the pore size of molecular sieve catalyst and the molecular size of bio-oil. The data of the pore size of molecular sieve catalysts are derived from crystal structural analysis, while the data of the molecular size of bio-oil are rather difficult to obtain. It is very necessary to estimate the molecular size of bio-oil. The pyrolysis of cellulose was carried out by prolysis-gas chromatography/mass spectrometry (Py-GC/MS), and the variation of the composition of cellulose pyrolysis bio-oil with the temperature was studied. The kinetic diameter of the components of cellulose pyrolysis bio-oil were calculated on the basis of Joback group contribution method, and the characteristics of molecular size distribution were analyzed. The results showed that cellulose pyrolysis bio-oil was mainly composed of anhydrosugars, furan derivatives and ketone compounds in the temperature range from 350℃ to 600℃. The kinetic diameter of cellulose pyrolysis bio-oil were mainly distributed in the range of [0.500, 0.600) nm. When the pyrolysis temperature increased from 350℃ to 600℃, the peak area of bio-oil molecules with the kinetic diameters in the range of [0.550, 0.600) nm decreased from 88.72% to 64.53%, and the peak area of bio-oil molecules with the kinetic diameters in the range of [0.500, 0.550) nm increased from 2.88% to 21.95%. For the preparation of high-quality liquid fuels by catalytic cracking of cellulose, molecular sieve catalysts ZSM-5, ZSM-11 and IM-5 with pore size of 0.500 to 0.600 nm can be selected.