Abstract: To cope with the potential security hazard of current inverted index, we propose an improved secure index structure.We encrypt the terms of the inverted index to defend the semantic analysis attack.We omit the position offset and frequency of terms to resist the statistics attack.We use block-based encryption to resist the known plaintext attack.We also introduce two-level index structure to balance security and efficiency of the index.Based on the proposed secure index structure, we design the encryption strategy for the secure index and give a multi-level key management policy.