The invention discloses a biaxial test device, comprising: a bracket, a container, a sample setting assembly and a press, etc. Before use, the sample is installed between two limit plates. On the basis of applying confining pressure, the press is started, and the pressure rod moves vertically downward and applies a vertical downward load to the sample. When the sample is subjected to the vertical downward load, axial compression and radial deformation will occur. At this time, the radial deformation of the sample on both sides of the limit plates is limited, and expansion occurs on both sides without the limit plates. Subsequently, under the joint action of the confining pressure and the limit plates, the sample undergoes plane strain. Since the container and the limit plates are both in a transparent state, the side section state of the sample when plane strain occurs can be well observed. At the same time, the multiphase mixed fluid in the mixing container penetrates into the rock sample inside the sample and flows out through an output pipeline and is tested and analyzed by external equipment, so that the potential influence of multiphase seepage on the mechanical behavior of rock mass can be studied under plane strain conditions.