Abstract: Based on theoretical analysis of the extrusion process of biomass materials and the data from actual measurement, a mathematical model of the extruding apparatus of optimum design was established. From some design requirements constraint conditions were derived, and the main parameters of the apparatus such as outside diameter of the screw, depth of the channel of the screw pitch and so on were taken as the design variables. This design problem can be abstracted from mathematics as a constrained nonlinear programme which contains seven variables and eighteen inequalities. The optimum structural parameters can be obtained by means of optimization technique through the method of repeated iteration and verified by experiments. The results showed that the values from optimum calculation were basically close to the real working condition. By using the apparatus developed, the production ability has been increased by 32. 9%-40%, and consumption decreased by 30. 8%. Technical characteristics of the apparatus were improved obviously.Besides, applying the optimum software for solving this nonlinear programme the extrusion working process can be simulated as well. When changing operation conditions or taking different materials (i . e. different input data) , a group of optimum structural parameters and related data such as the conduction ability, consumption of power and pressure can be obtained.