The invention discloses an 
optical flow gradient amplitude characteristic-based subtle 
facial expression detection method and relates to 
processing for identifying a graphic recording carrier. According to the method, face edges are obtained by means of fitting according to face key points, and a face 
region of interest is extracted; an 
optical flow field between face image frames in a 
video sequence is extracted by using a FlowNet2 network; the 
optical flow gradient amplitude characteristics of the face 
region of interest are extracted; characteristic distances are calculated and processed, and 
noise elimination is carried out; and therefore, subtle 
facial expression detection based on the optical flow gradient amplitude characteristics is completed. According to subtle facial expressiondetection in the prior art, subtle 
facial expression motion cannot be captured in extracted face 
image motion features; the features contain excessive interference information, and as a result, the subtle facial expression detection is susceptible to the influence of head deviation, blinking motion, accumulated 
noise and single-frame 
noise in feature 
distance analysis. However, with the method adopted, the above defects in the prior art can be eliminated.