A method for acquiring motion information, comprising: performing, by an excitation device, a mechanical vibration excitation on a medium to be detected to make the medium generate a mechanical vibration wave and the mechanical vibration wave propagates in the medium, wherein a wavefront of the mechanical vibration wave reaches different positions at different times along a propagation direction to form motion information of the mechanical vibration wave, and the motion information of the mechanical vibration wave is a correspondence between the different positions and the different times; performing dynamic imaging for the medium by an imaging device using an ultrasonic detection signal comprising the motion information of the mechanical vibration; subjecting the ultrasonic detection signal to a frequency domain transformation in an imaging time dimension by using a characteristic that the mechanical vibration wave generates Doppler effect, to obtain a frequency domain signal; wherein, the frequency domain transformation is performed by Fourier transform or singular value decomposition; removing a signal that is outside of a defined mechanical vibration propagation velocity range of the wavefront from the frequency domain signal to obtain a processed signal; and obtaining a position-time diagram of a wavefront of the mechanical vibration wave using the processed signal, wherein the position-time diagram represents the motion information of the mechanical vibration wave; wherein the removing a signal from the frequency domain signal to obtain processed signal comprises: performing a filtration or a feature value selection on the frequency domain signal to obtain the processed signal, wherein a parameter of the filtration is set according to the defined mechanical vibration propagation velocity range of the wavefront, and the feature value selection is set according to the defined mechanical vibration propagation velocity range of the wavefront. performing, by an excitation device, a mechanical vibration excitation on a medium to be detected to make the medium generate a mechanical vibration wave and the mechanical vibration wave propagates in the medium, wherein a wavefront of the mechanical vibration wave reaches different positions at different times along a propagation direction to form motion information of the mechanical vibration wave, and the motion information of the mechanical vibration wave is a correspondence between the different positions and the different times; performing dynamic imaging for the medium by an imaging device using an ultrasonic detection signal comprising the motion information of the mechanical vibration; subjecting the ultrasonic detection signal to a frequency domain transformation in an imaging time dimension by using a characteristic that the mechanical vibration wave generates Doppler effect, to obtain a frequency domain signal; wherein, the frequency domain transformation is performed by Fourier transform or singular value decomposition; removing a signal that is outside of a defined mechanical vibration propagation velocity range of the wavefront from the frequency domain signal to obtain a processed signal; and obtaining a position-time diagram of a wavefront of the mechanical vibration wave using the processed signal, wherein the position-time diagram represents the motion information of the mechanical vibration wave; wherein the removing a signal from the frequency domain signal to obtain processed signal comprises: performing a filtration or a feature value selection on the frequency domain signal to obtain the processed signal, wherein a parameter of the filtration is set according to the defined mechanical vibration propagation velocity range of the wavefront, and the feature value selection is set according to the defined mechanical vibration propagation velocity range of the wavefront. <EOS>