Vehicular platoons are among the most advanced driving assistance systems that may generate considerable fuel savings and increase traffic efficiency. However, communication between vehicles in a platoon is always vulnerable to eavesdropping due to the broadcast nature of wireless channels. To address this issue, we investigate security issues from the physical layer perspective for multi-hop vehicular platooning. The dynamic shielding secured transmission scheme is proposed to guarantee the confidential transmission of private information. Specifically, the neighboring vehicles can alternately act as friendly shielders, which transmit jamming signals to interfere with the eavesdroppers without knowing the channel state information, and thus the secrecy capacity would increase. The mathematical derivation of secrecy capacity is obtained for performance analysis. Meanwhile, the numerical results verify the properties, efficiency, and adaptability of the proposed scheme.