Abstract: The changes of hydrocarbon composition of hydrocracking tail oil in different reaction stages are discussed using Saudi Arab VGO as a feedstock. Paraffin content of hydrocracking tail oil increases with conversion rate rising. The proportion of paraffins in tail oil remains basically no change when the conversion rate is lower than 58.7% and decreases when the conversion rate is higher than 58.7%. Analysis suggests that ring-opening of cyclic hydrocarbon, off side chain reaction and the reaction products into the light component are the main causes for the changes. Therefore, the paraffin content of tail oil increases with the improvement of cyclic hydrocarbon ring opening ability of the catalyst.