Abstract: The pressure distribution inside disengager determines the flow path of the gas and steam, feed speed and the height of material seal leg. To well understand the distribution pattern inside the FCC disengager, a numerical simulation of oil gas flow in a full-size disengager of fluid catalytic cracking unit (FCCU) was conducted by Fluent 6.2 software. The Reynolds stress transport turbulence model (RSM) was used. The commercial disengager of 1.40 Mt/a FCCU was used as the computational geometry model, including the two stage separators and the built-in riser. The simulation results indicate that the space in the disengager can be divided into three regions according to the pressure: high-pressure region inside the primary cyclone separator and riser; medium-pressure region inside the disengager and low-pressure zone inside the secondary cyclone separators. The pressure inside the diplegs of the secondary cyclone separator is the lowest in the disengager, so it is necessary to establish an effective material seal of catalyst particles to balance the negative pressure and lock the air for nesting, or will cause the phenomenon of air leakage.