This paper proposeds a switching method for autonomous vehicles in a platoon to switch from cooperative adaptive cruise control(CACC) mode to adaptive cruise control(ACC) mode under communication interruption. A grouping optimization method for the platoon is established based on communication capabilities for vehicles subject to communication interruption. The cruise mode of vehicles subject to communication interruption is switched from CACC to ACC, but the time headway in CACC remains the same. Then, a time headway switching method based on minimum safe distance is proposed to ensure the safety of the platoon by combining the collision avoidance theory and Lagrangian median theorem. The mixed CACC-ACC controllers are designed to ensure the stability of the platoon (there are both CACC and ACC vehicles in it) after two switchings by combining the final value theorem and the frequency domain method. Finally, simulation results show the effectiveness of the proposed method by comparing with the existing methods.