The invention discloses an automatic express cargo size detection system. The automatic express cargo size detection system comprises a detection platform and an image obtaining device arranged on thedetection platform; and the image obtaining device is used for obtaining a projected image of a to-be-detected cargo under a fixed light source and is further connected with an image analyzer, and the image analyzer is used for processing and analyzing the obtained image so that the size of the to-be-detected cargo can be calculated. The image obtaining device is arranged for obtaining the imageof the cargo which is placed on the designated spot of the detection platform, and the image analyzer is used for processing and analyzing the image; the position, relative to the detection platform,of the image obtaining device is fixed relatively, and the direction and position of the light source are also fixed, so that the size of the projection of the obtained image is directly in positive correlation with the size of the cargo, the size of the cargo can be relatively correctly worked out based on corresponding parameters so that follow-up sorting can be purposely and accurately conducted on cargoes, and thus, automatic sorting of cargoes in the logistics express industry can be possible.