A signal transmission method suitable for DDR for driving a connecting pad includes a level shifting circuit including up and down level shifters, a buffer circuit including up and down buffer units, and an output circuit. The level shifting circuit, disposed between a DDR operating voltage and a ground voltage, receives an input signal in a first operating voltage equal to the ground voltage and a second operating voltage smaller than the DDR operating voltage. The up buffer unit is disposed between the DDR operating voltage and a first reference voltage, and the down buffer unit is disposed between the ground voltage and a second reference voltage equal to the second operating voltage. The up and down level shifters adopt IO devices, and other components adopt core devices. The first reference voltage is a difference between the DDR operating voltage and the second reference voltage.