In　order　to　apply　the　unified　strength　theory　taking　tension　as　positive　to　the　rock　and　soil　engineering　customarily　taking　compression　as　positive,　the　experimental　conditions　of　simple　compression　and　simple　tension　tests　are　substituted　in　the　expression　of　the　unified　strength　theory,　and　the　relative　parameters　are　determined,　hence　an　expression　of　unified　strength　theory　is　derived　for　rock　and　soil　materials,　where　compression　is　regarded　as　positive,　and　the　difference　of　tension　and　compression　strength　of　rock　and　soil　materials　and　the　effect　of　intermediate　principal　stress　are　taken　into　account.　The　presented　unified　strength　theory　is　then　introduced　to　establish　an　elasto-plastic　softening　constitutive　model　of　triaxial　state　in　strain　space　for　a　sort　of　diatom　soft　rock.　A　numerical　simulation　is　carried　out　along　with　the　test　data　of　the　diatom　soft　rock,　and　the　result　shows　that　the　simulation　of　the　unified　elasto-plastic　softening　constitutive　model　coincides　well　with　the　test　and　describes　the　strain　softening　accurately.