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The plane strain apparatus is an important instrument for studying the strength and deformation characteristics of soil with a specific stress path under plane strain condition and it is indispensable for theoretically studying rock and soil mechanics. After reviewing and summarizing the development history of the plane strain apparatus, we improved the pressure chamber, stress measuring mechanism, loading mechanism, and created a pressure chamber that can simulate a plane strain state. This improvement was based on the true triaxial apparatus with vertical rigidity and flexible loading in two orthogonal horizontal planes developed by the Xi'an University of Technology. We conducted plane strain tests of intact loess and remolded loess under different water contents and consolidation confining pressures. The rationality and reliability of the modification were verified by comparing the stress-strain curves and strength parameters obtained from plane strain and conventional triaxial tests, such that the evolution law of intermediate principal stress and the intermediate principal stress coefficient in the plane strain shear process was preliminarily revealed. Subsequently, the coefficient of intermediate principal stress at failure was found to be between 0.13 and 0.53. Thus, the effective modification of the plane strain apparatus can provide an experimental means for studying the strength and deformation characteristics of soil under plane strain condition. [ABSTRACT FROM AUTHOR]