In order to evaluate the design schemes for heat exchanger and other mechanical equipment，based on the extension theory，a multilevel assessing index system for heat exchanger design schemes was proposed，a multilevel matter-element model for the design scheme was established，and a multilevel assessing method for the design scheme was proposed. A comprehensive evaluation of shell and tube heat exchanger was realized，the objectivity and validity of the result were confirmed，and the result shows that the method is effective and reliable.