A　focus　optical　system　was　designed　to　focus　the　rectangular　spot　of　2　750　W　direct　laser　diodes　stack　for　laser　material　processing.　First,　the　beam　transformation　principle　was　analyzed　theoretically.　The　invert　Kepler　telescope　system　principle　was　applied　to　expand　and　collimate　the　laser　beam　in　the　slow　axis　direction.　The　fast　and　slow　axis　beams　were　synchronous　focused　by　focus　lens.　The　formula　of　focus　spot　size　in　the　fast　and　slow　axis　directions　was　deduced,　and　the　theoretical　value　of　focal　depth　was　calculated.　The　influence　factors　of　the　focus　spot　size　were　analyzed.　Second,　LD　stack　and　the　whole　optical　system　were　simulated　by　ZEMAX　software.　After　the　ray　tracing,　the　beam　variation　form　and　simulate　focus　spot　were　obtained.　Finally,　the　experiment　was　performed,　a　focus　spot　whose　size　was　1.5　mm×4.0　mm　with　250　mm　focal　length,　and　18.6　mm　focal　depth　was　obtained.　The　merit　and　demerit　of　the　whole　focus　optical　system　and　improve　orientation　in　the　future　were　discussed.　This　focus　method　could　obtain　long　focal　length　and　high　power　density　focus　spot,　which　can　meet　the　light　source　requirement　of　laser　cladding　and　surface　treatment.　©　2018,　Science　Press.　All　right　reserved.