原因是没有设置
~~~
<activity
android:name="com.znykt.knowall.WellcomeActivity"
android:theme="@android:style/Theme.Black.NoTitleBar.Fullscreen" >
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
~~~
