Soft　body　armor　can　effectively　safeguard　lives　from　stabbing　attacks　and　they　are　mainly　composed　of　fibrous　composites.　Currently,　the　evaluation　of　protective　performance　for　soft　body　armor　is　mainly　conducted　by　experiments　or　numerical　simulations,　with　theoretical　methods　have　been　rarely　mentioned.　In　this　study,　a　spring-mass　based　theoretical　model　has　been　established　to　predict　the　stabbing　resistant　performance　of　a　soft　body　armor　comprising　aramid　with　resin　or　unidirectional　UHMWPE　composites.　The　theoretical　results　agree　well　with　the　experimental　results　where　the　error　in　maximum　impact　force　of　the　knife　is　within　3.2%　and　the　displacement　within　1.7%.　The　layers　of　penetration　are　also　identical.　By　only　inputting　hardness　and　stiffness　of　the　specimen,　the　theoretical　model　can　efficiently　provide　the　performance　of　a　soft　body　armor　composed　of　fiber　composite　material　under　different　impact　energy　levels.　With　the　aid　of　this　theoretical　model,　one　can　optimize　the　composition　of　a　soft　body　armor　to　achieve　minimum　weight　and　thickness　in　practical　applications.