The concentration profiles of jet gas are investigated in the feed injection zone of a cold-model FCC riser by using a hydrogen trace technique.Experimental results demonstrate that six types of jet gas concentration profiles can be used to describe the mixing process of the jet gas with pre-lift gas and catalyst particles in the feed injection zone.The six types are a distinct M-shaped profile for no mixing region，an indistinct M-shaped profile，a distinct three-peak profile，an indistinct three-peak profile and an one-peak profile for mixing region，and a annular-core profile for full mixing region.The heights for regions of initial and full mixing reduce when decreasing the jet gas velocity or increasing the pre-lift gas velocity. Furthermore，the momentum ratio，Mj/Mr，is introduced to describe effects of gas-solid physical properties，operating conditions，and equipment configuration on the jet gas concentration distribution.The heights for regions of no mixing，mixing and full mixing between jet gas with pre-lift gas and catalyst particles are found to be 0—0.375 m，0.375—0.525 m and 0.525—0.675 m when Mj/Mr≤0.29，0—0.525 m，0.525—1.225 m and 1.225—1.375 m when Mj/Mr>0.54，and 0—0.375 m，0.375—0.875 m and 0.875—1.075 m when Mj/Mr between 0.29 and 0.54，respectively.