In　order　to　depict　the　softening　characteristics　of　soft　rock　effectively,　firstly,　a　formula　for　the　unified　strength　theory　taking　the　compression　as　positive　is　established.　This　theory　considers　the　strength　difference　of　tension　and　compression　and　the　mediate　principal　stress　effect.　So　it　is　suitable　for　geotechnical　materials.　Secondly,　the　limit　line　in　π　plane　of　this　theory　is　derived.　Thirdly,　a　three-dimensional　unified　elastic　viscoplastic　softening　constitutive　model　is　derived　based　on　the　limit　line　in　π　plane　and　the　model　of　Yin-Graham.　Parameters　of　the　model　are　determined　with　the　relevant　triaxial　tests　on　a　sort　of　diatom　soft　rock　which　is　distributed　in　Nodo　Peninsula　in　Japan,　belonging　to　the　late　tertiary　Miocene,　and　formed　of　debris　diatom,　clay　and　volcanic　ash　by　depositing,　and　usually　distributed　in　inshore　regions.　And　finally,　numerical　simulation　is　done.　The　simulated　results　of　the　three-dimensional　unified　elastic　viscoplastic　softening　constitutive　model　are　very　close　to　the　experimental　results,　proving　that　the　model　can　be　used　to　accurately　simulate　the　softening　behaviors　of　soft　rock.