The importance of data security is discussed when charging facilities for electric vehicles are supervised through the public network. Data security methods are compared and a suitable one is selected by analyzing the normal security mechanism and data communication characteristics of the charging facilities， as well as the demands of engineering practice. As the embedded system is generally used in the charging facilities to reduce cost and improve reliability， an improved method for the normal one-time pad mode is put forward， in which the key pool technology is used to simplify encryption software， improve efficiency， and increase the difficulty in deciphering ciphertext by increasing redundant data bytes and transmitting message with fixed length on each packet.