A memory management method, a memory storage device and a memory control circuit unit are disclosed. The method includes: performing a first write operation to store first data to a first physical unit; recording first unit management information corresponding to the first write operation, wherein the first unit management information reflects a usage order of first used physical units, and the first used physical units include the first physical unit; performing data merge operation to copy at least a part of data stored in the first physical unit to a second physical unit; and after the data merge operation is performed, recording second unit management information according to the first unit management information, wherein the second unit management information reflects a usage order of second used physical units. The second used physical units include the second physical unit but do not include the first physical unit.