Crystalline　waveguide　prepared　by　adhesive-free　bonding　technique　(AFB)　has　good　mode　limiting　effect.　In　this　paper,　considering　the　mode　competition,　the　waveguide　core　size　under　the　single　mode　condition　is　calculated,　which　can　increase　the　core　size　1.79　times　comparing　with　the　traditional　result.　A　400　mu　m　x　320　mu　m　large　core　size　Yb:YAG　crystalline　waveguide　is　used　to　build　a　crystalline　waveguide　laser,　and　output　the　beam　quality　is　achieved　with　M-2　=　1.22　x　1.05.　The　experimental　results　of　near　diffraction-limited　output　verify　that　the　calculating　method　is　reliable.