The surrounding rock of underground engineering such as hydraulic tunnels is in a plane strain state. In order to identify the characteristics of rock plane strain state, a series of rock mechanic tests are conducted based on discrete element method (DEM). Firstly, the significance and problems of rock plane strain compression test (PSCT) are described; Secondly, parameters of a granite are calibrated by the lab uniaxial test results, and then the characteristics of the rock plane strain state are analyzed from the data and microcrack development; Finally, the threshold for strain confining device of the rock PSCT are further explored. The results show that: (1) 3 characteristic data points are given by analyzing the axial stress, confining wall stress and crack development trend of rock samples under PSCT, and the staged process of rock from damage to fracture is revealed; (2) The 3 characteristic points of PSCT define the stage process of the crack development: the cracks develop from scattered to localized, and then deepen the connection until the samples breaks; (3) It is suggested that the control threshold of the confine device should be the unidirectional strain corresponding to the stress peak point under uniaxial compression. The characteristic points coordinates of the half plane strain state obtained under the control of this threshold can basically coincide with the characteristic points of the complete plane strain state.