Ethereum smart contracts face serious security problems, which not only cause huge economic losses, but also destroy the Ethereum credit system. To solve this problem, code obfuscation techniques are applied to smart contracts to improve their complexity and security. However, the current source code obfuscation methods have insufficient anti-decompilation ability. Therefore, we propose a novel bytecode obfuscation approach called BOSC based on four kinds of bytecode obfuscation techniques, which is directed at solidity. The experimental evaluation results show that, after the bytecode obfuscation, the failure rate of decompilation tools is over 99% and only a small amount of gas is consumed.