A plane - strain apparatus for investigating the deformation and failure features of rock was altered and constructed on a passive stiff-frame concept, further, a new device inducing confinement on both sides of the prism specimen was designed and built, which can not only impose varying confining pressure on the specimen, but dynamically monitor the change in confinement due to deformation of the specimen during testing. Theoretical analysis and test results show that for rock specimens with Youngt modulus below 17.4 GPa, the apparatus can provide an acceptable planestrain condition and be of practicability.