<p>The adoption of electronic contracts has significantly improved the efficiency of contract signing and management, especially in cross-border trade. However, it also raises challenges related to security and fairness. Traditional solutions are vulnerable to signature forgery and third-party collusion, while blockchain-based solutions, despite their traceability and tamper-resistance, face issues such as privacy leakage and high costs. To address these challenges, this paper proposes a multiparty fair contract signing model that combines off-chain protocols with on-chain smart contracts. The model uses shared keys to encrypt contract content, preventing sensitive information leakage; employs off-chain signature aggregation to reduce communication and storage costs; and implements multi-signature verification and a penalty mechanism via smart contracts to ensure fairness and security. Experimental results demonstrate that the proposed solution outperforms existing methods in privacy protection, signing efficiency, and economic feasibility, offering a high-performance, secure, and fair solution for multiparty contract signing.</p>