An extended evaluation method of mechanical design schemes was proposed by means of extension theory. Based on the matter-element model and extended relation degree, the extended evaluation method can carry out the decision-making of machine design schemes. The matter-element model of the expected design scheme and the matter-element models of alternative design schemes were established according to specialists' practical experiences, and then the quality order of schemes was evaluated by the extended relation degree. The relation degree is the measurement index to evaluate how the design scheme is close to the expected one (ideal scheme). The bigger the relation degree is, the better the close degree between the design scheme and the expected scheme is. Finally, the rationality of evaluation in mechanical design schemes was demonstrated through illustrations.