The invention discloses a deep learning small target detection method and device based on cascade fusion and an attention mechanism, and the method comprises the following steps: S1, inputting a to-be-detected image, and carrying out the preprocessing of the to-be-detected image; s2, performing feature extraction on the preprocessed image by using a deep convolutional neural network based on cascade fusion and an attention mechanism, extracting to obtain target image features, performing feature fusion by a feature cascade fusion layer based on a cascade feature fusion structure, enabling the spatial attention mechanism layer to obtain a semantic mask of the small target area, fusing the semantic mask with the original features channel by channel, and outputing extracted target image features; and S3, carrying out prediction and post-processing on the extracted target image features to obtain a final target detection result, and outputting the final target detection result. The invention can achieve small target detection based on deep learning, and has the advantages of being simple in implementation method, low in cost, high in detection efficiency and precision, flexible in operation and the like.