The invention provides a data transmission method, a data transmission device and a data transmission system. The method comprises the steps of generating an asymmetrical secret key pair including a first public key and a first private key, and sending a data request carrying the first public key to a server side; receiving a ciphertext and a second public key sent by the server side, wherein the second public key is the public key in the asymmetrical secret key pair acquired by the server side, the asymmetrical secret key pair acquired by the server side also comprises a second private key, the ciphertext is information generated by encrypting a seed parameter used for generating an offline payment code by using a sharing secret key, and the sharing secret key is the secret key by using a preset secret key negotiation algorithm according to the second private key and the first public key; and generating the sharing secret key by using the secret key negotiation algorithm according to the first private key and the second public key, using the sharing secret key to decrypt the ciphertext, thus acquiring the seed parameter. According to the method, the device and the system provided by the invention, security of the data during a whole transmission process is ensured, and decryption and encryption efficiency is also improved.