To verification the reliability and the effectiveness of the RCS of large electric size target by using scaling model, derivation of the constraint conditions to get the same electromagnetic field of actual target by scaled model on the basis of the theory of electromagnetic field. Verified two standard objects which with same sharp and different scale by darkroom measurement and software simulation(FEKO,HFSS) respectively, and analyzed the RCS of a certain plane and scaling model by FEKO and HFSS simulation. It well solved the problem of large scale or super large complex target RCS difficult to obtain or cannot be.