A subsampling motion detector configured to detect motion information of an object under measurement receives a first wireless radio frequency (RF) signal and transmits a second wireless RF signal, the first wireless RF signal being generated by reflecting the second wireless RF signal from the object. The subsampling motion detector includes a controllable oscillator outputting an oscillation signal, wherein the first wireless RF signal is injected to the controllable oscillator for controlling the controllable oscillator through injecting locking. The subsampling motion detector further including a subsampling phase detector (SSPD) generating a control signal according to the oscillation signal generated by the controllable oscillator and a reference frequency, the SSPD outputting the control signal to the controllable oscillator for controlling the controllable oscillator, the oscillation signal of the controllable oscillator being locked to a multiple of the reference frequency and the control signal representing the motion information of the object.