The frame adopts goose neck structure. The vertical member used high intensity and alloy steel. Welded to H pattern with automatic submerged arc; the height of work flat is low. It can carry digger, road roller, bulldozer, large tools and facilities. The low flatbed semi-trailer can classify to rigidity suspension system structure (one line with axle), the frame can be flexible type, tire-exposing type, flatbed type and concave-line type etc.