A data writing method for a memory storage device having physical unit unions is provided, wherein each of the physical unit unions includes upper physical units and lower physical units. The method includes partitioning the physical unit unions into a storage area including a data area and a spare area; configuring logical units for mapping to the physical unit unions of the data area; and receiving update data from a host system. The method also includes: selecting several physical unit unions from the spare area as buffer physical unit unions; writing the update data only to a part of each of the buffer physical unit unions; and moving the update data from buffer physical unit unions to the storage area by using a copy procedure. Therefore, the time of performing a write command can be shorten and the lifespan of the memory storage device can be prolonged effectively.