Usually the gearbox casing of twin screw pulping extruder was designed by experience and greater weight. In order to reduce the casing weight under the premise of ensuring its stiffness and strength, firstly the casting lower box and the weakest axle in the casing were analyzed with finite element method by ANSYS software. And then the optimum design was carried out to take the minimum volume as an objective function. Finally, the welding casing has been designed as considering the manufacturing cost. The results show that the weight of the casting lower box after optimization is 22.95% lower than that of the experience design, and the weight of the welding part is 19.09% lower than that. The work has certain significance for optimum design of gearbox casing.