The invention provides a data compression method, an access method and a system for a block chain database. The data compression method includes: extracting user address data from the value domain data of the key-value entry stored in the key-value database, and counting the average number of user key-value entries; when the average user key-value entry number is higher than a predetermined threshold, Creating a secondary hash table to store user address data in each entry, so that the entries with the same user address data correspond to the same secondary hash table; and incepting the partial bytes of the complete key domain data of each entry to generate simplified key domain data; Replacing the user address data in the value field data of each entry with a pointer to the correspondingsecondary hash table to generate simplified value domain data; generating a primary hash table based on the simplified key domain data and the simplified value domain data to replace the initial hash.The table uses the linear data table to store the key-value data in which the hash map address conflict and the first-level hash table simplify the key value conflict, and the data includes the complete key domain data and the value domain data.