A frame for a dump semi-trailer comprises an upper frame and a lower frame, and the compound mode of the upper frame and the lower frame is that two longitudinal beams of the gooseneck part of the upper frame are corresponding to and are arranged beyond two longitudinal beams of the gooseneck of the lower frame; the gooseneck part of the upper frame is arranged above a bearing pull blade of the lower frame and the gooseneck support of the lower frame; the upper plane of the upper frame is higher than the upper plane of the upper wing plate of the gooseneck longitudinal beam of the lower frame. The frame for the dump semi-trailer with the structure of the invention can reduce the height of the bearing plane of the trailer, thereby reducing the height of gravity center of the whole vehicle after being loaded, avoiding turn-over when unloading, increasing the volume of the trailer without changing the total length and width of the trailer, loading more goods and increasing conveying efficiency.