In order to meet the requirement of the grass braid mechanization technology in the pasturing area such as the Qingzang Plateau, a grass braid forming device was designed with the function of twisting and plying referring to the principle of Siro-spinning. The mechanical analysis showed that grass braid forming process was affected by axial movement speed, rotating speed of twisting roller, radius of twisting roller and feed quantity. The final twist and twist angle of grass braid were only relevant to the rotational speed of twisting roller and the linear velocity of pulling rollers, but had nothing to do with the spinning time and length of twisting zone. As a new method, the maximum tension force that grass braid could bear was taken as assessment index of grass braid strength. The prototype experiment was carried out with oat straw with the moisture content of 61.05%. It showed that the productivity of the forming device was 427kg/h, the working power was 0.428kW, and the bulk density of formed grass braid was 143.3kg/m3,respectively, which met the design requirements of grass braid forming device. The results of tension tests showed that the maximum tension force of the grass braid was positively correlated with the twist angle, and the maximum force reached 300~350N when the displacement was equal to half of the length of grass. The fracture of grass braid was due to the slip between two bunches of grass, which had nothing to do with grass fracture. This method and experiment provided theoretical support for grass braid forming technology.