The invention relates to the technical field of semi-trailer machining, in particular to a spraying device of a semi-trailer body. The spraying device comprises two first spraying mechanisms and a plurality of second spraying mechanisms, and the second spraying mechanisms are detachably arranged between the two first spraying mechanisms. Each first spraying mechanism comprises a first spraying frame, a configuration box and a first spraying device. Each second spraying mechanism comprises a second spraying frame and a second spraying device arranged on the second spraying frame. The sides, away from the configuration boxes, of the first spraying frames are detachably installed on the second spraying frames. Every two adjacent second spraying frames are detachably arranged. The configuration boxes provide paint for spraying of the first spraying devices and the second spraying devices. The invention further provides a spraying method for coating by utilizing the spraying devices. The whole semi-trailer body is sprayed at the same time, and therefore the coating efficiency is improved. Mechanical spraying is achieved, and the defect that manual spraying influences the spraying efficiency, the spraying quality and the personal health of operators is overcome.