The　angle　compensation　method　is　adopted　to　detect　sloshing　waves　by　laser　diffraction,　in　the　case　that　the　wavelength　of　the　sloshing　waves　is　much　greater　than　that　of　the　incident　light.　The　clear　diffraction　pattern　is　observed　to　be　of　asymmetry,　involving　orders,　position　and　interval　of　the　diffraction　spots　that　are　discovered　during　the　light　grazing　incidence.　It　is　found　that　the　larger　the　angle　of　incidence　is,　the　more　obvious　the　asymmetry　is.　The　higher　the　negative　diffraction　orders　are,　the　smaller　the　intervals　between　spots　are.　On　the　contrary,　in　the　positive　region,　the　higher　the　diffraction　orders　are,　the　larger　the　spot　intervals　are.　The　positive　interval　is　larger　than　that　of　the　same　negative　diffraction　order.　If　the　incident　angle　reaches　1.558　rad　in　the　experiment,　all　positive　diffraction　orders　completely　vanish.　Based　on　the　mechanism　of　phase　modulation　and　with　the　Fourier　transform　method,　the　relations　between　the　incident　angle　and　position,　interval　spaces,　and　orders　of　diffraction　spots　are　derived　theoretically.　The　theoretical　calculations　are　compared　with　the　experimental　data,　and　the　comparison　shows　that　the　theoretical　calculations　are　in　good　agreement　with　the　experimental　measurement.