The invention provides a data compression method, storage method, an access method and system in a key-value database. the data compression method comprising: extracting user address data from value domain data of a plurality of stored key-value entries in a key-value database, and counting an average user key-value entry quantity based on the user address data; When the number of value entries ishigher than the predetermined threshold, creating a secondary hash table in the key-value database to store the user address data in the initial hash table of each key-value entry, so that the key-value entry with the same user address data corresponds to the same secondary hash table; replacing the user address data in the initial hash table of each key-value entry with a pointer to the user address data in the corresponding secondary hash table, and storing the initial hash table after the user address data is replaced as a first-level hash table of each key-value entry in the key-value database.The method and the system of the invention can improve the memory access performance in the block chain database.