试了一天终于搞定了，主要有两点：
1.viewport不要设width=device-width，应使用以下代码
<meta content="initial-scale=1.0, maximum-scale=1.0, user-scalable=0" name="viewport">
<meta content="yes" name="apple-mobile-web-app-capable">
2.设置一张640*1096的启动图。图片记得调成640*1096，放在Images/Default-568h@2x；代码中添加下面这行
<link href="Images/Default-568h@2x.png" rel="apple-touch-startup-image" sizes="640x1096">
由于我的webapp不支持iPhone横屏，支持iPad横屏，所以，一共七张Default.png，三张iphone的，四张ipad的。图片的分辨率千万别错，不然不识别。
七张图分辨率分别是：
Default.png 320*460
Default@2x.png 640*920
Default-568h@2x.png 640*1096
DefaultIpadPortrait.png 768*1004
DefaultIpadPortrait@2x.png 1536*2008
DefaultIpadLandscape.png 1024*748
DefaultIpadLandscape@2x.png 2048*1496
对应的代码如下：
<link rel="apple-touch-startup-image" href="Images/Default.png" media="(device-width: 320px)" sizes="320x640"/>
<link rel="apple-touch-startup-image" href="Images/Default@2x.png" media="(device-width: 320px) and (-webkit-device-pixel-ratio: 2)" sizes="640x920"/>
<link rel="apple-touch-startup-image" href="Images/Default-568h@2x.png" media="(device-width: 320px) and (-webkit-device-pixel-ratio: 2)" sizes="640x1096"/>
<link href="Images/DefaultIpadPortrait.png" media="(device-width: 768px) and (orientation: portrait)" rel="apple-touch-startup-image"/>
<link href="Images/DefaultIpadPortrait@2x.png" media="(device-width: 768px) and (orientation: portrait) and (-webkit-device-pixel-ratio: 2)" rel="apple-touch-startup-image"/>
<link href="Images/DefaultIpadLandscape.png" media="(device-width: 768px) and (orientation: landscape)" rel="apple-touch-startup-image"/>
<link href="Images/DefaultIpadLandscape@2x.png" media="(device-width: 768px) and (orientation: landscape) and (-webkit-device-pixel-ratio: 2)" rel="apple-touch-startup-image"/>
