6. The method according to claim 4 , wherein, in the recognition stage, a table of possible maximum gray scales and minimum gray scales against different thicknesses for each category of cargoes is constructed; and a minimum possible weight and a maximum possible weight of a cargo are obtained by reference to a gray scale of an image and the table of maximum gray scales and minimum gray scales.
6. The method according to claim 4 , wherein, in the recognition stage, a table of possible maximum gray scales and minimum gray scales against different thicknesses for each category of cargoes is constructed; and a minimum possible weight and a maximum possible weight of a cargo are obtained by reference to a gray scale of an image and the table of maximum gray scales and minimum gray scales. 7. The method according to claim 6 , wherein, after the recognition stage, the method further comprises one of more of the following: 1 ) displaying different cargoes using different colors according to categories of the cargoes, informing an operators how many categories the cargoes cover, and which category each category of the cargoes pertains to; 2 ) displaying at a click position in a certain region a category of cargoes and a minimum possible weight and a maximum possible weight of the category of cargoes according to a click operation of an operator; 3 ) if a result is different from a manually input category, highlighting the difference with a color on the scanned image; and 4 ) automatically analyzing a Manifest to obtain a cargo category and corresponding weight information, and if a result is different from an automatic analysis result, highlighting the difference with a color on the image.