Taking an aluminum alloy thinwalled workpiece as the research object, its surface roughness was improved by optimizing the three cutting parameters, ie. cutting depth, feed rate and cutting speed. Aiming at the semifinishing and finishing stages, 16 groups and 25 groups turning experiments were designed according to the empirical parameters, and corresponding surface roughness were measured. Relationship between surface roughness and the three cutting parameters were established, and the optimum cutting parameters which could minimize the surface roughness were solved. Using the optimized cutting parameters, defect rate and reject rate of the thinwalled workpiece were decreased from 16.03% and 14.23% to 11.25% and 7.5% respectively. The results show that the optimized cutting parameters improve qualified rate of the thinwalled workpiece significantly.