This paper analyzes the error types for segmented reflector lamp, and puts out an optimal design method for segmented reflector based on error analysis. The paper establishes optimization model for segmented reflector considering the error analysis in manufacturing process of segmented reflector. Based on this optimization model it could give out shapes of reflector patches for segmented reflector which is low sensitive to the errors, and direct the plan process efficiently and get out the final reflector consistent with the designed one, which is important for auto lamp design.