Log-based security audit plays a key role in the safe operation of the system and evidence collection, Log auditing can effectively detect relevant threats and provide warnings in time, it can also trace the traces of entity behavior through logs. The log audit of the power grid is currently handled by the audit center. The audit center records the entire operation process and provides audit record query. But the research on the audit center system found that the audit center lacks security protection for the logs. Aiming at the above problems, a log anti-tampering system based on block chain technology is proposed, which uses block chain storage structure and underlying cryptography technology to ensure that the log cannot be tampered with. The test results show that after the log is protected by this method, it takes a lot of cost to tamper with the log without being detected, which effectively ensures the security of the log.