A method for writing data into a flash memory unit includes: when writing data into the flash memory unit for the n-th time, determining a data polarity of an n-th data bit to be written into the flash memory unit; selectively injecting an n-th electrical charge amount into a floating gate of the flash memory unit according to the data polarity of the n-th data bit; when writing data into the flash memory unit for the (n+1)-th time, determining the data polarity of an (n+1)-th data bit to be written into the flash memory unit; and selectively injecting an (n+1)-th electrical charge amount into the floating gate of the flash memory unit according to the data polarity of the (n+1)-th data bit. The (n+1)-th electrical charge amount is not equal to the n-th electrical charge amount, and n is a positive integer not less than 1.