Abstract: Axial and radial solid concentration profiles inside a novel riser reactor featured by bottom enlargement were investigated and the characteristics of gas-solid microflow behavior in the diameter-enlarged section were analyzed in a cold model apparatus. The results show that the solid concentration distributions are characterized by two regions: dense bottom region and dilute upper region with a reduced radial non-uniformity index along the axial direction. Compared with the bottom region of the conventional riser, higher solid concentration and intermittency index with more uniform radial distribution in the diameter-enlarged section are observed, demonstrating a stronger gas-solid interaction and a smaller radial gradient of the gas-solid microflow behavior. The gas-solid microflow behavior inside the diameter-enlarged section performs similarly to that in turbulent flow and is significantly affected by the structure of the reactor. The fluctuations of solid concentration in the diameter-enlarged section are characteristic of large amplitude and high frequency, and the distributions of dilute phase and dense phase are relatively uniform, which intensifies the gas-solid contacting and mixing process.