In view of the problems in the current trace-free blockchain covert communication technology， such as the single dimension of carrier embedding and the risk of network ecological pollution， a trace-free covert communication model based on smart contracts was proposed. The proposed model is composed of three algorithms to work together and is able to explore characteristics of the smart contract opcode sequence deeply. Firstly， the preprocessing algorithm was used by both communicating parties to reconstruct the contract opcode characteristics and construct a triplet feature sequence. Secondly， the binary-coded original message was concealed in the reconstructed sequence through the associated parameter-assisted mapping， so as to generate a binary random key used for transmission. And， the risk of key leakage in the transmission process was reduced effectively by combining the custom transmission protocol. Finally， the key was input into the reverse parsing algorithm by the receiver to realize the accurate restoration of the information. Experimental results show that the proposed model has an average embedding capacity of 5 042 bits per contract and an embedding efficiency of 6 MB/s， and the randomness of the generated key and the irrelevance of the generated key to the original information are proved by the National Institute of Standards and Technology （NIST） test and the mutual information test.