In order to solve the problem that the existing automatic image cropping algorithm mistakenly cuts off the whole person or part of the body parts of the main figure, which leads to the missing of the key information of the image and the poor composition of the image, this paper proposes a real-time automatic image cropping method based on portrait detection. This method divides image cropping into two stages: portrait detection and intelligent composition. It aims to take the extracted portrait coordinate information as the input of the intelligent cropping algorithm, and then combines the composition rule based on aesthetics to automatically compose the picture, so as to ensure the integrity of the portrait information in the cropping result and improve the composition beauty of the picture. The experimental results show that the lightweight detection network modified on the basis of Center-Net in this paper reduces the computation amount by 86% and improves the accuracy by 3.34%, which is convenient to apply the cropping algorithm to mobile devices. The FPS of the whole cropping algorithm reaches 77, and the cropped portrait information is complete, and the overall composition of the picture is improved.