This paper presents a mechanical reliability study of 3-dB waveguide hybrid couplers in submillimeter and terahertz bands. To show the necessity of improving the mechanical properties of the coupler’s branch in submillimeter and terahertz bands, a comprehensive study regarding the displacement of hybrid branch variation with varying width-length ratio and height-length ratio has been completed. In addition, a modified 3-dB waveguide hybrid coupler is designed and presented. Compared with the traditional branch structure, the proposed hybrid consists of a modified middle branch with circular cutouts at the top and bottom on both sides instead of the traditional rectangle branch, which increases the branch size and improves its mechanical reliability while achieving the same performance. Simulation results show that the deformation of the modified hybrid branch is 22% less than those of other traditional structure designs under the same stress. In practice, a vibration experiment is set up to verify the mechanical reliability of hybrid couplers. Measurement results show that the experiment deteriorates the coupling performance. Experimental results verify that the performance of the novel structure coupler is better than that of a traditional structure branch hybrid coupler under the same electrical properties.