The result of triaxial test shows that soft rock has notable strain softening characteristic

and the stress-strain curves present different types in the normally consolidated field and over-consolidated field. In normal consolidated field the softening degree of soft rock is slight relatively; but in over consolidated field

when axial stress comes to peak value

the softening speed increases rapidly with the increasing of strain. In order to simulate the stress-strain curves of this soft rock

the constitutive equation of consolidated undrained triaxial stress state expressed in strain space is deduced. This constitutive equation is used to simulate the stress-strain curves of soft rock

and in simulation different hardening functions

including a new proposed one

are applied. The simulation result shows good agreement with test result. It demonstrates that constitutive model established in strain space has good ability to describe the strain softening; and the new hardening function shows good applicability for normal consolidated soft rock under consolidated undrained state.