Abstract: The numerical simulation of the gas flow field around the trickle valve of cyclone in FCC disengager was conducted by CFD software FLUENT6.2, and the erosion cause of trickle valve plate surface was analyzed. The calculation results indicate that the oil gas is entrained into the dipleg through the clearance between the valve plate and the valve port due to the negative pressure of the dipleg. The amount of oil gas increases with increasing the valve plate opening and the negative pressure. The oil gas carries catalyst particles to impact on the valve plate, which is the main reason of the erosion. The upflow of oil gas through the dipleg also reduces the separation efficiency of the cyclone separator.