The invention relates to a pepper leaf disease detection method based on improved AlexNet, and the method comprises the steps: building a disease data set, carrying out the data enhancement of the disease data set, carrying out the disease type classification of the image data of the disease data set, marking a corresponding label, building a model data set, and carrying out the detection of a pepper leaf disease. Dividing image data of the model data set into a training set, a verification set and a test set in proportion; secondly, constructing a convolutional neural network model, performing feature extraction on an AlexNet model in the convolutional neural network model, setting a multi-scale convolution kernel for a first convolutional layer, removing a full connection layer, replacing the full connection layer with a global average pooling layer, adding a BN layer into each convolutional layer, then setting hyper-parameters, and obtaining a multi-scale convolution kernel; and training the AlexNet model by using the training set. According to the improved AlexNet-based pepper leaf disease detection method provided by the invention, the model can be reduced, the identification precision can be improved, and the detection speed can be improved.