Though　3D　numerical　simulation　of　laminar　flow　and　comparison　of　the　simulated　results　of　slotted　fin　surface　with　plain　plate　surface,　this　paper　optimizes　the　designs　of　slotted　fin　of　tube-fin　heat　exchanger　used　in　evaporator　of　air　conditioning　under　low　velocity　and　Re　number　(inlet　velocity　is　1~3　m/s,　and　Re　number　is　about　901~2702).　It　is　found　that,　within　the　inlet　velocity　range　studied,　the　performance　curve　of　slotted　fin　and　plain　plate　heat　exchanger　has　inevitably　a　crossing　point　whose　Re　number　is　called　turning　Re　number;　if　the　Re　number　is　lower　than　this　turning　Re　number,　the　performance　of　plain　plate　surface　is　better　than　that　of　slotted　fin　surface;　however,　if　the　Re　number　is　higher　than　this　turning　Re　number,　the　larger　the　velocity,　the　better　the　slotted　fin　surface　performance　compared　with　the　plain　plate　fin　surface.　Moreover,　if　the　strip　numbers　of　slotted　fin　surface　is　decreased　according　the　rule　of　＇front　sparse　and　rear　dense＇,　the　performance　of　heat　exchanger　can　be　enhanced　effectively　and　the　turning　Re　number　can　be　decreased　evidently.　In　addition,　the　results　of　the　present　study　once　again　show　the　consistent　relationship　between　the　higher　heat　transfer　rate　of　the　slotted　fin　surface　with　their　better　synergy　between　velocity　and　temperature　gradient.