Data transmission method, terminals and system thereof

The embodiments of the invention disclose a data transmission method, and a system and related devices thereof. The method comprises the steps that: a first terminal uses a preset encryption key to encrypt a data transmission key and sends a communication connection request carrying the encrypted data transmission key to a second terminal; the second terminal uses a preset decryption key to decrypt the encrypted data transmission key, and obtains and stores the data transmission key; the second terminal sends a connection response message to the first terminal for establishing communication connection between the first terminal and the second terminal; based on the communication connection with the second terminal, the first terminal uses the data transmission key to encrypt data to be transmitted and sends the encrypted data to be transmitted to the second terminal; and the second terminal uses the data transmission key to decrypt the encrypted data to be transmitted and obtains the data to be transmitted. A