The invention discloses a neonatal pain expression classification method based on a CNN (
convolutional neural network). The method comprises the following steps: firstly, neonatal pain expression images are acquired, the images are classified by professional 
medical staff according to calm, cry, slight pain and intense 
pain level by level, and a neonatal pain expression image 
library is established; secondly, the CNN containing one data layer, three convolutional 
layers, two full connection 
layers and one classification layer is constructed; then samples in the neonatal pain expression image 
library are taken as 
data input of the CNN, iterative training is performed on the CNN with a 
back propagation algorithm, and optimizing training is performed on global parameters, so that a 
network output loss function value is reduced and converged; finally, neonatal pain expression test samples are input and then recognized and classified with the CNN, expression recognition of neonatal infants in calm, cry, slight pain and intense pain states is realized, and one novel method and way is provided for evaluating neonatal pain degree.