For wall jet model with co-flow used in the downburst, four turbulent models were used to analyze the flow characteristics of the wall jet with co-flow, including the average wind velocity profile and Reynolds stress at different developing stages. The results show that the prediction of the modified Reynolds stress model (RSM) is in close agreement with the experimental data. The modified RSM is effective and accurate for the wall jet with co-flow, and it is also used to investigate the effects of ratio of the wall jet bulk velocity and co-flow velocity on the wall friction coefficient and mean wind velocity profile. It is found that when β increases from 0.1 to 0.3, the mean velocity increases with the velocity ratio, the maximum velocity degradation becomes slower gradually, the wall friction coefficient decreases more rapidly, and the interaction between inner layer and outer layer is decreased.