[Purposes] The paper aims to improve the efficiency of the off-ramp section of the connected expressway, reduce the risk of traffic accidents and ensure the order of diversion vehicle traffic. [Methods] A cooperative lane changing strategy is proposed to address the traffic disorder caused by the lane changing behavior of connected automated vehicle (CAV) upstream of the off-ramp. Taking into account both traffic efficiency and comfort, a collaborative CAV dynamic speed optimization model is constructed with the objective of minimizing the weighted sum of the average speed and average acceleration of all CAV during the speed, acceleration, acceleration vibration, and the longitudinal distance between the start of the lane change and the diversion point as constraints to systematically optimize the speed of each CAV in each period. The Gurobi optimizer is used to solve the cooperative control model, and a simulation scenario is built using SUMO to evaluate the cooperative control effect. [Findings] The results show that the proposed synergistic method can improve the average vehicle speed by up to 17.7%, reduce the total delay by more than 75.9%, and improve the average acceleration change rate by more than 9.3% under different total flow and diversion ratios compared with the uncontrolled case the diversion ratio is certain, for a certain total flow rate, the higher the total flow rate, the better the improvement of the average velocity and the average acceleration rate of change the minimum longitudinal distance constraint of safe lane change requirement, the exit ramp section has the highest traffic efficiency. [Conclusions] Under different total flow and diversion ratio, the synergistic strategy can create lane change gap for lane change vehicles, improve traffic efficiency and enhance passenger comfort.