Abstract: The protolytic cracking of Daqing vacuum gas oil (VGO) catalytic cracking process was studied by conducting multiple series of experiments from the low reaction conversion to high conversion over acid catalyst in a small fixed fluid bed (FFB) unit. A concept of the secondary protolytic cracking is proposed for VGO catalytic cracking process. The reactants of the secondary protolytic cracking are mainly paraffins in naphtha that are readsorbed on the acid catalyst and then cracked subsequently. The primary reason that causes the secondary protolytic cracking is the change of selective adsorption of the intermediate products on catalysts at the late reaction stage. The secondary protolytic cracking is not very sensitive to the reaction temperature. The portion of the secondary protolytic cracking in the whole protolytic cracking process is 60% for Daqing VGO at 500℃.