The gas-phase flow hydrodynamics in the feed injection zone of a fluidized-bed reactor is investigated by the steady-state helium tracer technique. The helium concentration is calculated and analyzed. The results show that the helium concentration distribution can be subdivided into three subregions along the axial direction, which are the constant region, jet region, and the diffusion region. On this basis the nozzle jet diffusion length is proposed and modeled to account for the length of the jet region and diffusion region. By comparison of the nozzle jet diffusion length under different operating conditions, the conclusion that the nozzle jet diffusion length increases with increasing nozzle jet velocity but decreases with increasing catalysts circulation rate is obtained. Besides, empirical correlation for nozzle jet diffusion length is given with high prediction accuracy.