Ethereum, the largest blockchain for running smart contracts, has been widely used, especially in financial and cryptocurrency exchange applications. Among them, Tornado Cash is a typical financial application that protects the privacy of users with anonymous transactions. However, users need to pay prohibitively high gas (transaction fees for smart contract calls) for anonymous transactions, which hinders Tornado Cash from wide applications. To address this issue, we introduced a new approach that shifts the high gas-consuming operations on smart contracts to local users. Furthermore, we use zero-knowledge proofs to ensure the operations are properly executed. The smart contract only needs to verify and update the results, which significantly reduces the gas fees of Tornado Cash. To validate our approach, we implemented a prototype and showed that our proposed method could save more than 61% of gas consumption of current operations while maintaining the privacy feature of Tornado Cash. Finally, we discussed further applications and open problems of our approach.