The utility model provides a back-frame strengthening structure used for semi-trailers, and comprises a tubular corner-post component, a crossbar component and a profiled angle steel (8) arranged at the crossing angle formed by the tubular corner-post component and the crossbar component; the profiled angle steel (8) is tri-folded bending-edge shaped, and the upper bending-edge part(81) thereof is inserted in the crossbar component and is fixed and connected with the inner side surface of the crossbar; the two bending-edge parts of the vertical part (82,83)of the profiled angle steel are inserted in the tubular corner-post component, and are respectively fixed and connected with two face-to-face inner side surfaces(B1, B2) of the tubular corner-post component; thus, the back-frame strengthening structure used for novel refrigerated trailers of the utility model has a simple structure and a good technic realization. The utility model can greatly improve the strengthening effect of the prior art, and can guarantee the insulation performance of the refrigerated trailers in largest degree.