Abstract: A novel optical element, combined triangular-section prism, for generating non-diffracting line-structured beam is proposed. This element is designed with gluing positive and negative isosceles triangular-section prisms. Its property is the same as the single positive isosceles triangular-section prism for beam transformation, and the equivalent bottom corner is decided by the bottom corner difference of positive and negative isosceles triangular-section prisms. Therefore, we can get smaller bottom corner by combining the positive and negative isosceles triangular-section prisms with comparatively large bottom corner for each triangular-section prism, and also get the non-diffracting line-structured beam with larger focus depth. The technical problem of the small bottom corner which is hard to machine is solved. The formation mechanism of the non-diffracting line-structured beam is analyzed by geometrical optics, and the relevant parameters are calculated. The intensity distribution is simulated by diffraction and interference theories. The results show that the nondiffracting line-structured beam is formed with large depth of focus at normal incidence of plane waves.