The residence time distributions of upward and downward gas injections in a fluid catalytic cracking riser were investigated by large-scale cold model experiments. The tanks-in-series model was employed to describe the local mixing of feed jets with catalyst particles. The model parameter M at various axial and radial measuring points was obtained by using the helium tracer method. On this basis, M was fitted with the jet-catalyst flow momentum ratio and the axial height. By knowing M of the full mixing zone of the riser, this method calculates the influence height of jets above the injection point. Results show that the downward feed offers a fuller mixing environment for jets and catalysts in their initial contact region. Besides, the influence height above the nozzles in the downward injection scheme is over 50% shorter than that of the upward jet case. This study confirms that the downward jets can provide a more ideal environment for jet-catalyst mixing.