A write control method, an associated data storage device and the controller thereof are provided. The write control method includes: receiving one or more commands from a host device and executing the one or more commands, and performing determining operations to generate determining results; in response to the determining results, starting performing write collection; collecting and handling one or more write commands to buffer data of the one or more write commands through a buffer memory, and performing other determining operations to generate other determining results; in response to the other determining results, according to a quantity of one or more collected and handled write commands, determining whether data thereof has filled up a write buffer region of the buffer memory; and in response to the data having filled up the write buffer region, flushing the write buffer region to write the data therein into a non-volatile memory.