A data accessing method using dynamic speed adjustment with aid of a thermal control unit, and associated apparatus such as memory device, memory controller, etc. are provided. The data accessing method includes: utilizing a thermal control unit to start monitoring temperature at a predetermined intra-controller location of the memory controller; in response to at least one accessing request from a host device, controlling a transmission interface circuit to perform data transmission between the host device and the memory controller at an original communications speed, for accessing data in the NV memory; in response to the temperature being greater than a first temperature threshold, detecting an increment of the temperature between a first start time point and a first end time point; based on at least one first predetermined rule, determining a first communications speed according to the increment; and switching from the original communications speed to the first communications speed.