The invention discloses a data transmission method and system. The method comprises the following steps: when a server is connected with a terminal, the server generating a first connection request instruction, and encrypting the first connection request instruction by use of a secret key preset in an encryption chip so as to obtain a first ciphertext for sending to the terminal; the terminal decrypting the first ciphertext by use of the secret key so as to obtain the first connection request instruction; if the first connection request instruction is a valid instruction, the terminal generating a second connection request instruction, and encrypting the second connection request instruction by use of the secret key so as to obtain a second ciphertext for sending to the server; the server decrypting the second cipertext by use of the secret key so as to obtain the second connection request instruction; and if the second connection request instruction is valid, the server encrypting transmission data needing to be transmitted by use of the secret key and sending the encrypted transmission data to the terminal. The security of data transmission between the server and the terminal is guaranteed, and data transmitted between the server and the terminal is prevented from being illegally changed.