This article studies the relation between fabric tightness and shielding effectiveness (SE) of blended electromagnetic shielding fabric (BESF). The SE of different tightness BESFs is tested using a waveguide testing method. Experimental analyses show that the SE of BESF associates to the tightness linearly, the SEs of same tightness fabrics are consistent under same metal fiber contents, same weave types and different yarn linear densities condition, and the SEs of different type fabrics are inconsistent under same tightness, same metal fiber contents and same yarn densities condition. Moreover, a theory about the relation between the SE and the tightness determined by adjacent yarns states is proposed according to the electromagnetic theory. A number of computation equations about tightness boundary determination are given, which would provide reference for the BESF design, manufacture and testing.