The gas tracer technology was employed in this research to investigate the jet diffusion and its mixing with particles in the gas?solid riser. Studies were finished in a large scale cold model riser. In order to investigate the influence of different types of jets, both upward and downward injections were used during experiments. By introducing the jet characteristic concentration, the radial distributions of jet after injecting into the riser were obtained. Results showed that the distribution of jet concentration was more uniform for the case of downward injection. By detecting the tracer gas at various axial and radial points, the local residence time distributions of feed jet were obtained. On this basis, the dimensionless variance was calculated and comparison between upward and downward injections was made. It showed that the values of variance and their fluctuations were large at most axial cross sections when the jet was upward, indicating some serious local backing-mixing of feed jet. If the direction of injection changed to downward, the calculated value of variance was large under the feed nozzles, while the value of variance was small and its radial distribution was uniform at the locations above nozzles. This result meant that the jet-solid mixing flow can change into the likely plug flow from a likely full mixed flow easily and quickly under the influence of downward injection. Based on experimental results, the residence time variance was fitted with operating conditions and axial height. Empirical formulas for the cases of both upward and downward injections were obtained. Finally, comparing fitted results with the variance value in the riser full mixing zone, the influence height of jet in the riser was calculated. Results showed that the influence height of feed jet can be shortened by 50% if the downward injection was chosen.