A data writing method, a memory control circuit unit and a memory storage apparatus are provided. The method includes: determining whether receiving a predetermined command from a host system. The method also includes: if receiving the predetermined command from the host system, writing at least one buffer data from a buffer memory into a first physical erasing unit, selecting at least one second physical erasing unit from the physical erasing units, and writing at least one valid data of the at least one second physical erasing unit into the first physical erasing unit in response to the predetermined command.