Vehicular platoon is one of the most advanced driving assistance systems with the potential to bring considerable savings in fuel 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 study the security issues from the physical layer perspective for the multi-hop vehicular platooning. The dynamic shielding secured transmission (DSST) scheme is proposed to guarantee the confidential transmission of private information. Specifically, the neighboring vehicles can take turns acting as friendly shielders, which transmit jamming signals to interfere with the eavesdroppers without knowing channel state information (CSI), thereby increasing the secrecy capacity. The mathematical derivation of secrecy capacity is conducted for performance analysis. In addition, the numerical results verify the properties, efficiency, and adaptability of the proposed scheme.