A signal calibration method that includes the steps outlined below is provided. A phase of an under-test signal generated by a memory controller is set to initiate a calibration process. A low-power status control command is issued by transmitting signals that include the under-test signal generated by the memory controller to a memory unit to switch the memory unit to a low power status, the low-power status control command forcing the under-test signal to toggle. A read command is issued by the memory controller to the memory unit for reading data. When the responded data does not match the predetermined data, the phase of the under-test signal is determined to be within a timing margin by the memory controller. When the responded data matches the predetermined data, the phase of the under-test signal is determined to be not within the timing margin by the memory controller.