When QC12Y16×4000 hydraulic swing beam shear was used to cut plate, with the length of plate increasing, the plate was moved. In severe cases, the displacement difference was 7 mm. It was shown that the plate had moved seriously in cutting process. By analyzing plate moving phenomenon, the reason was that holddown pressure was less. By redesigning the holddown foot and the hydraulic system, the experimental result shows that the improving plan is reasonable, cutting plate precision reaches and exceeds the shear machine national standard I level precision. User requirement is satisfied.