Improving　diode　laser　fiber　coupling　efficiency　is　the　key　to　improving　the　efficiency　of　fiber　lasers.　We　have　demonstrated　a　new　expression　for　the　diagonal　beam　quality　of　a　rectangular　spot　beam　of　a　diode　laser,　which　reflects　the　actual　value　approached.　By　considering　the　application　of　fiber　coupling　and　the　astigmatism　characteristics　of　the　diode　laser,　we　have　introduced　an　astigmatism　factor,　using　the　extreme　value　and　the　approximation　of　the　overall　divergence　angle　in　the　offset　direction.　From　this　we　have　obtained　a　new　universal　expression　for　the　diagonal　beam　parameters　of　the　diode　laser.　The　degree　of　matching　between　the　theoretical　value　and　the　experimentally　measured　value　was　greater　than　99.7%.