In this paper, a terahertz beam splitter based on an open ring structure is proposed. Its structural unit is a typical metal-dielectric-metal structure. The top metal pattern has the form of an open ring, and the bottom layer is a continuous metal plate. An 8×8 phase gradient metasurface is formed by changing the opening size and radius of the opening ring. When the terahertz wave is vertically incident along the -z and +z axes, the electromagnetic wave will be divided into four beams that are symmetrical along the x and y axes but have different energy distributions. The two incident modes can obtain two different beam splits. At 0.7THz, when the terahertz wave is vertically incident along the -z and +z axis directions, the beam splitting ratios are 0.8∶1 and 1.9∶1, respectively, realizing the tuning of the beam splitting ratio. This beam splitter has the advantages of small size and low cost, and it can be applied in the fields of terahertz communication, terahertz imaging, and terahertz stealth.