Beserch on selecting traverse mode by hollow waveguide tube in cavity and a new laser with "waveguide/free-space"compound cavity are reported. The key for achieving "multimode oscillation and single mode output"is the compound cavity where large enoungh multimode oscilating volume exists in the free-space gain region and the lowest waveguide mode output controled by hollow waveguide tube. A theoretical analysis was made of the principle and foreground of compound cavity which was confirmed by experimental results.