After i update my ipad 2 to ios7 , auto brightness not working.

Auto-brightness works for me in iOS 7.

Read this: http://support.apple.com/kb/HT5506

Also note that auto brightness does not dim the screen immediately when the ambient light sensor detects dimming light, if it did the screen would dim every time your hand covered the sensor. If you move to low light the dimming feature will work when you turn off/on the screen.