It's a key of software evolution to achieve available method of software change-trace.A whole trace-process framework of requirement change is drawn based on three tiers which are propagation modeling

propagation path

and trace approach of requirement change.Record method about propagation of function-requirement change is addressed based on trace-matrix of change-source

of change-object and of change component.Implementation of change-trace based matrixcalculation is described

and a simple cost decision method of software change-implementation is brought forward.It is advantage to implement software maintenance as well as software evolution.