Biomass is one of the promising alternative materials to solve the energy and environmental crisis. Fast pyrolysis is one of the most economical and commercially realizable technologies to convert biomass to useable fuels and chemicals. To improve the liquid products, co-pyrolysis with polypropylene (PP) over a mesoporous catalyst MCM-41 was studied in this paper. Thermogravimetry Fourier transform infrared spectroscopy (TG-FTIR) and pyrolysis gas chromatography mass spectrometry (Py-GC MS) were used as the main analysis methods to study the mass loss and detailed products of the co-pyrolysis. The mass loss, main functional groups, and identified products of pyrolysis of cellulose, PP, and their mixture were analyzed and discussed. All of the TG, FTIR, and Py-GC MS data show that there is no significant synergism between cellulose and PP when simply mixing them, although the C/H-eff of the mixture increases from 0 to 1.3. However, the addition of MCM-41 bring significant synergism. The TG and differential thermogravimetry (DTG) data show that the co-pyrolysis with MCM-41 shifts the decomposition of PP to a lower temperature, which provides more overlap between cellulose and PP in the range of 300-400 degrees C. According to the FTIR spectra, CO, CO2, and carbonyl are also produced in the peak for pyrolysis of PP for the mixture together with MCM-41, which indicates the intermolecular synergetic reaction. Furthermore, the results from Py-GC MS show that olefins (43.9%), oxygenated compounds (24.8%, mainly alcohols), and aromatics (17.8%) are the main products of co-pyrolysis of cellulose and PP in the presence of MCM-41, while oxygenated compounds (82.2%, mainly saccharides), olefins (4.7%), and aromatics (1.1%) will be the main products without the catalyst. Olefins and alcohols are Much more than the calculated value, which is the main result of synergism. Alcohols are mainly produced from the radical from the cracking of PP combined with the hydroxyl radical produced from decomposition of cellulose. Olefins are produced from the interaction reaction (carbenium ion reaction and beta-scission) between the primary products of cellulose and the hydrocarbon pool reaction of primary products of cellulose and PP. The results of this study enhance the understanding of co-pyrolysis of cellulose and PP in the presence of MCM-41 and provide the possible pathway of modifying the special pyrolysis products in catalytic pyrolysis of biomass with polymers.