The invention relates to a block chain hidden information transmission method and system based on dynamic marking. The method comprises the following steps: counting data true probability distributionof an OP_RETURN field of a block chain protocol; generating a dynamic mark according to the negotiated seed and the data real probability distribution of the OP_RETURN field; and enabling the sendernode to fill the dynamic marks into the OP_RETURN fields of the two transactions, performing digital signature by adopting an elliptic curve cryptographic algorithm, and sending the two signed transactions to the block chain; enabling the receiver node to identify the two transactions according to the dynamic marks, performing elliptic curve cryptography algorithm random number leakage attack on digital signatures of the two transactions, calculating used random factors, deducing a private key, namely an encoded encrypted message, decoding the encrypted message and decrypting the encrypted message by using a secret key negotiated with the sender node. According to the invention, the undetectability and tracking resistance of the channel can be realized, the strong robustness is satisfied,and the risk of user privacy exposure is greatly reduced.