Cellulase Cel5A from Acidothermus cellulolyticus is an endoglucanase which features the retention mechanism for the cleavage of the beta-1,4-glycosidic bond. In this work, we investigated the detailed catalytic steps in the formation of two cellobiose units from the hydrolysis of a cellotetraose molecule using a combined QM/MM approach. The understanding of the catalysis process at the atomistic level may help further protein engineering research. Molecular dynamics, potentials of mean force (PMFs), and reaction path calculations confirmed that the hydrolysis of cellotetraose has a retention mechanism via oxocarbenium-like transition states for both the glycosylation and deglycosylation steps.