A numerical simulation model was applied to study the characteristics of an off-ramp junction’s traffic flow, based on analyzing the traffic flow of a Chinese urban expressway. In the course of this study, we constructed lane-changing rules with which to analyze special traffic flow characteristics. This construct was based on influences such as traffic volume parameters; the mainline rate; acceleration and deceleration values; and the length of the deceleration lane. Finally, we developed a new cellular automaton simulation model for the off-ramp junction of a Chinese urban expressway. Additionally, we analyzed the mainline traffic volume rate with different operation speeds, off-ramp junctions with different deceleration lane lengths, and the tendency chart of the mainline flow with different inflow probabilities. The results showed significant disturbance occurring between the outflow rates, mainline flow, and the operation speeds of mainline vehicles. By contrast, our new cellular automaton model was an efficient tool in the study of traffic flow from off-ramp junctions.