A secondary torsion extrusion device and method for refining crystal grains with a variable cross-section cavity, including: a punch part, a transmission mechanism, a power mechanism and a die part, wherein: the punch part cooperates with the die part, and the power mechanism It is connected with the transmission mechanism; the punch part includes the upper template and the upper mold, the concave mold part includes the upper cylinder, the upper plate and the main mold, the upper template is set on the upper mold, the upper mold is matched with the upper cylinder and the working state is in the hydraulic press Under the action of the slider, it enters the upper cylinder and the main mold. The upper cylinder is set on the upper plate, the main mold is set under the upper plate and connected with the upper cylinder, and the upper mold enters the upper cylinder when it is working. In the present invention, by changing the cross-sectional shape of the inner cavity of the rotating mold, the variable-section cavity mold is rotated at the same time of extrusion, and the material has both extrusion deformation and shear deformation caused by the rotation of the mold, thereby realizing severe plastic deformation and high forming force. Small, high deformation efficiency, can achieve good grain refinement effect.