A teaching aid for demonstrating the transverse mode structure in lasers is described. A novel device called "multi-dimension adjustable combined cat-eye reflector" has been constructed from easily available materials to form a He-Ne laser resonator. By finely adjusting the cat-eye, the boundary conditions of the laser cavity can be altered, which allows the transverse modes output by laser to be conveniently modulated. Equipped with a CCD imaging system, this apparatus allows more than 20 laser transverse modes to be visualized on the computer screen, provided a valuable teaching aid at an undergraduate level. (Contains 8 footnotes and 7 figures.)