The existing Eulerian video subtle motion magnification method does not take into account the automatic detection of motion information in the video, the appropriate parameters for motion information processing need to be selected in the realization of motion magnification, such as the filter cut-off frequency, and magnification. For the general video, these parameters usually could not be determined directly, but by trial-and-error. In this paper, an automatic amplification method of non-stationary subtle motion in video was proposed based on S transform. The instantaneous correlation parameters of band-pass filter were automatically determined based on S transform and the corresponding dynamic filter was designed. On this basis, an approach of automated subtle motion magnification was achieved. Firstly, the instantaneous frequency of subtle motion was obtained by S transform in video. Then the dynamic band-pass filter was used to process different frequencies at different times. Finally, the effective motion information which was filtered by band-pass filter was magnified to achieve subtle motion magnification. In addition, for the analysis of anti-noise performance, a method for evaluating the signal-to-noise ratio in video area was proposed. The experimental results show that the proposed method can automatically obtain the parameters such as filter and magnification according to the change of the frequency of the motion information when the actual video is amplified, without manual participation. After the motion amplification, the amplification effect of the moving target can be dynamically displayed. At the same time, the accurate dynamic filter can suppress noise to some extent, and makes the motion magnification effect better.