Aiming at the problem that it is difficult to determine the dosage of coagulant for algae outbreak in waterworks, a prediction method of algae coagulation removal rate based on deep learning is proposed. DenseNet convolution neural network and floc image are used to predict the algae coagulation removal rate, so as to adjust the dosage. The specific method is to coagulate the high algae water under laboratory conditions, and record the floc image and the corresponding removal rate range after coagulation. The floc image data set was constructed with the removal rate interval as a label, and the DenseNet-121 model was trained with this data set. The results show that the prediction accuracy of the trained model for the test set reaches 89.5%. Compared with VGG and RESNET convolution neural network, the DenseNet convolution neural network has higher recognition accuracy on the data set established in this paper and has obvious advantages over the other two models in identifying floc images with a removal rate of 60%~90%. At the same time, the recognition of Microcystis aeruginosa floc image outside the data set verifies the good generalization of the model.