Beyond the traditional use of frequency-shift keying (FSK) radar in tracking moving targets, this work further extends its capability in localizing stationary human subject based on the phase information extracted from the detected vital sign signals. Unlike the I/Q-trajectory-based range detection method that relies on trajectory analysis and therefore is prone to the effect of DC offset and trajectory distortion, the FSK-based technique used in this paper works in spectrum domain utilizing only the motion frequency of the target with a reliable signal-to-noise ratio (SNR). A 5.8-GHz industrial, scientific, and medical (ISM) band FSK radar with a frequency difference of 15 MHz is designed and implemented. Experiments demonstrate that, besides tracking walking human subject, the proposed FSK technique can successfully recover range information of a seated human subject at different locations. A preliminary investigation of the detection robustness and consistency is presented by measuring the range of a 4-mm peak-to-peak periodic motion generated by an actuator over a 90-s observation window.