The invention discloses a trimming shear device of a plate shearing machine, and relates to the technical field of plate processing, in particular to the plate shearing machine. The device comprises a fixed stander, wherein a linear motion unit and two rolling elastic supports are fixedly connected above the fixed stander; the linear motion unit is arranged between the two rolling elastic supports; a moving platform is slidably connected above the linear motion unit; the two sides of the moving platform are arranged above the two rolling elastic supports; and the moving platform is connected with a power device. Since the linear motion unit and the elastic rolling supports are adopted, the trimming accuracy, the trimming speed and the trimming range of a plate are greatly improved, the problem of deviation of plate feeding during trimming of a long plate is solved, and accurate long-distance feeding of the plate is realized. The device has a rational design, is convenient to operate, repair and maintain and is low in manufacturing cost.