A memory management method, a memory storage device and a memory control circuit unit are provided. The method includes: obtaining a usage status of a first physical unit of a rewritable non-volatile memory module for storing data from a host system; determining a first rule according to the usage status; and performing a first operation according to the first rule. The first operation includes: storing a first data from the host system into the first physical unit; and storing a second data from the rewritable non-volatile memory module into a second physical unit, where the first rule corresponds to a first ratio between a data volume of the first data and a data volume of the second data. Accordingly, the memory storage device can store external and internal data stably.