To obtain the flow details of gas phase, a three-dimensional numerical simulation of a novel industrial FCC disengager with flow partition cylinder quick separation system was conducted with RNG k-e turbulent model. The size of pressure balance tubes and the diameter of closing cylinder were mainly studied. The results show that the volumetric flow rate of stripping gas flowing into the exterior space of flow partition cylinder increases with increasing of balance tube size and decreasing of closing cylinder diameter. Closing cylinder prevents the oil gas from running into the disengager. The pressure balance tubes can accomplish the quick drawing-out of oil gas from the disengager by suction effect and make the oil gas total residence time fall to below 13 s. The extraction of stripping gas can be divided into two parts when the side length of pressure balance tubes is less than 310 mm, while the side length of which is more than 310 mm, a handful of oil gas will flow out of the separation system. When the diameter of closing cylinder is 3.8 m, more than 99% of stripping gas flows into the exterior space of closing cylinder.