The invention provides a molecular weight grading device for polyolefin, and relates to a grading device. The invention solves the problems of preparation of polyolefin with different molecular weights mainly through a polymerization method in the prior art, and fluctuation of temperature in the process of conventional polyolefin grading. The molecular weight grading device for polyolefin comprises: the top part of a heating box is provided with a three-mouth flask fixing opening; the side surface of the heating box is provided with a hose connecting opening and a three-mouth flask valve knob connecting opening; the bottom part of the three-mouth flask is provided with and communicates with a bottom tube; the bottom tube is provided with a three-mouth flask valve knob which penetrates through the three-mouth flask valve knob connecting opening of the heating box along a horizontal direction and is fixed; a vacuum filtering bottle is provided with and communicates with a hose which penetrates through the hose connecting opening of the heating box along a horizontal direction and is fixed; the upper mouth of the three-mouth flask upward penetrates through the three-mouth flask fixing opening located at the top part of the heating box and is fixed; the lower part of the bottom tube located at the bottom part of the three-mouth flask is provided with a funnel; and the bottom part of the funnel is provided with the vacuum filtering bottle. The invention relates to the molecular weight grading device for the polyolefin.