Abstract：Plane strain test is an early-developed，relatively widely-studied，yet immature test. The fact that this method is still applied recently in many fields shows that it is still an important test method. This paper first reviewed plane strain test research history at home and abroad，and features and defects of each main type of the apparatus were explained，showing that it is still necessary to make improvement on the test accuracy and reliability of the apparatus to solve the friction problem，confinement effect and bedding error etc. On such a purpose，this paper makes improvement in the apparatus structure，measuring method，control accuracy and data treatment when carrying out plane strain test. Friction is minimized by adopting a device for loading intermediate principal stress，whose centre is kept at the same level in axial compression and accuracy in measuring the vertical stress is improved. Rubber sleeve pasted with silicon grease is used to solve the paradox of friction problem and bedding error. A gap sensor is used to improve measurement of the deformation of specimen，and measured data is treated in a way bedding error can be eliminated. More important，control accuracy of intermediate strain is studied by compensating the deformation of rubber sleeve，which is important in improving the test accuracy. Experiment on sand is carried out after checking the influence of each factor on the test result，deformation property and strength parameter is compared with the result of traditional test apparatus. The result in turn shows the accuracy and reliability of the improved apparatus. Then，based on failure criterion obtained by the test apparatus and equation between principal stresses under plane strain condition，the relationship between strength parameters of plane strain state and trixial compression state is derived；comparison is also made with other references.
