When I watch satellite channel through AV mode suddenly TV changes to normal channel so I press AV button again in the remote control to change to AV mode again why

Are you using a universal remote? For example, a lot of times a universal remote will find itself in "TV" mode. If you try to press the channel button to change channels, you will probably end up changing the TV's channel instead of the satellite box's channel.

Make sure the remote is in SAT mode before attempting to change channels.