The　influence　of　tool　rake　angle,　feed　rate　and　back　cutting　depth　on　the　surface　roughness　and　surface　waviness　of　KDP　crystal　were　studied　when　processing　KDP　crystal　in　the　way　of　horizontal　fly　cutting.　Comparing　with　the　influence　laws　of　vertical　single　point　diamond　fly　cutting,　we　analyzed　the　respective　advantages　of　the　two　kinds　of　cutting　processing　methods　and　summarized　the　similarities　and　differences　of　the　influence　rules　that　cutting　parameters　on　surface　quality　under　the　two　ways.