A data storing method, a memory control circuit unit and a memory storage device are provided. The method includes: receiving a first write command from a host system; determining whether to write a first data corresponding to the first write command by using a first mode or write the first data by using a second mode according to an available buffer memory state; writing the first data into a first physical erasing unit among a plurality of physical erasing units by using the first mode when the first data is determined to be written by using the first mode; and writing the first data into a second physical erasing unit among the physical erasing units by using the second mode when the first data is determined to be written by using the second mode.