The mechanism of cellulose pyrolysis with CaO is studied using a reactive force field in molecular dynamics simulations (ReaxFF MD). Through the analysis of the changes in the products and bonds generated during the simulation process, the cellulose pyrolysis characteristics, both with and without CaO, under different reaction conditions (temperature, heating rate, and Ca/C mass ratio) are obtained. The detailed reaction pathways of cellulose pyrolysis with CaO under a heating rate of 20 K/ps are revealed. When the temperature or heating rate is increased, cellulose pyrolysis is promoted. The addition of CaO significantly promotes cellulose pyrolysis. With the increase of the Ca/C mass ratio, more oxygen-containing groups are fixed by CaO. The total number of C-O bonds in the system following the addition of CaO is lower than that in the pure cellulose pyrolysis system. After CaO was added, the number of CO and H2O molecules decreased. One of the main products of cellulose pyrolysis, glycolaldehyde (C2H4O2), with high oxygen contents, also decreased. The mechanisms are also given. The results of cellulose pyrolysis with CaO in ReaxFF-MD simulations are consistent with the results of an online pyrolysis-single-photon ionization time-of-flight mass spectrometry study on camphorwood pyrolysis. This study suggests that ReaxFF MD simulations are helpful to study the detailed and dynamic chemical reactions of biomass pyrolysis with alkaline earth metals.