A device and a method for controlling a bit density of a magnetic card (2) are provided. Said control device comprises a stepper motor (1) providing power to drive the magnetic card (2) or a recording magnetic head (15), a transmission mechanism transferring power to the magnetic card (2) or the recording magnetic head (15), a CPU (10) sending magnetic recording information, a track data encoder (13) receiving a magnetic writing pulse signal (CLK1) and generating magnetic writing data in synchronization with the magnetic writing pulse signal (CLK1), and a magnetic head driving circuit (14) outputting a magnetic writing control signal based on the magnetic writing data. The frequency of the magnetic writing pulse signal (CLK1) and the frequency of a driving pulse signal (CLK0) of the stepper motor are in a linear relationship. Said control method is a method for controlling a bit density.