A new automatic optical polarization demultiplexing method for NRZ-OOK polarization division multiplexed (PDM) system is proposed. The detected power differences of both the direct current (DC) and the radio frequency (RF) are used as control signals which are sensitive to the angles between the polarization controller (PC) and the polarization beam splitter (PBS). The characteristics of those control signals are thoroughly analyzed, and the best RF detection window is calculated out through system simulation.