This　study　attempts　to　modify　the　unified　strength　theory　by　considering　compression　as　a　positive　load　in　geotechnical　engineering.　It　also　aims　to　establish　a　unified　elastoplastic　strain　softening　constitutive　model　which　can　accurately　describe　the　strain　softening　behavior　of　one　kind　of　soft　rocks　distributed　in　Japan.　The　hardening　function　parameters　of　the　unified　elastoplastic　strain　softening　constitutive　model　are　determined　from　experiments.　In　addition,　numerical　simulations　of　this　model　are　performed　to　compare　the　pre-peak,　post-peak　and　the　residual　strengths　of　soft　rock　predicted　by　this　study　and　experimental　results.　Simulation　results　demonstrated　that　the　proposed　constitutive　equations　in　strain　space　can　well　describe　the　softening　behavior　and　accurately　predict　the　peak　and　residual　strengths　of　soft　rock.　While　the　proposed　equation　is　applicative　for　normally　consolidated　state　and　overconsolidated　state　according　to　the　simulation　results.