A query string for an encrypted database storing a plurality of encrypted data records is received from a requestor. The query string is segmented to obtain at least one word. The at least one word is encrypted with the irreversible encryption algorithm to obtain at least one encrypted word. At least one first encrypted item with a co-occurrence weight higher than a preset threshold based on the at least one encrypted word and a co-occurrence statistics model is acquired. The co-occurrence statistics model is built to provide co-occurrence weights, each indicating a probability that the at least one encrypted word appears in a first encrypted data item of the plurality of encrypted data records. At least one second encrypted data item corresponding to the at least one first encrypted data item is acquired from the plurality of encrypted data records.