1. A method for monitoring breathing activity, adapted to an electronic device, comprising: receiving a time series of incoming frames of a monitored subject and splitting each incoming frame in the time series of incoming frames into a plurality of blocks, wherein each of the blocks of the time series of incoming frames with a same block position forms a time series of incoming blocks; computing a motion vector of each of the blocks and projecting the motion vector of each of the blocks onto a plurality of non-parallel axes; assigning a sign to each of the projected motion vectors with respect to each of the axes, wherein the sign is either positive, negative, or zero; merging the projected motion vectors of each of the time series of incoming blocks with respect to each of the axes respectively into a corresponding one-dimensional (1D) waveform and converting the 1D waveforms into a frequency domain; locating an observe area associated with the monitored subject in the time series of incoming frames according to the converted 1D waveforms; and determining a breathing frequency of the monitored subject corresponding to the observe area and thereby determining whether or not the monitored subject is breathing abnormally.