The present invention provides a data storage device including a flash memory and a controller. The controller receives a read command arranged to read data from a host, determines a plurality of read tasks by analyzing the read command, and determines task time of each of the read tasks according to the number of the read tasks and an I/O latency time. In a first read task of the read tasks, the controller reads a part of the data and transmits the read part to the host, and executes a first maintenance process according to a predetermined condition, wherein the predetermined condition includes a remain time and the remain time is the task time minus the time spent by the first read task.