I want excluded a part from top level assembly without effecting sub assembly.if i use that sub assembly in different model i don't want to be excluded.

My suggestion is to create a separate configuration in the subassembly with the part excluded.  Otherwise, move the part to the end of the BOM in your drawing and hide the row. 

When you say excluded, do you mean hidden or suppressed OR not there in BOM?

For hidden, create a display state.

For suppressed, create a configuration

No there in BOM: Sub assembly appears as single component in top level BOM unless you have promoted the child of sub assembly OR use the indented/flat type of BOM. In this cases, you can create a sub assy configuration with that part suppressed OR as Solid Air mentioned, move the row to top and hide.

You can hide an individual component of a sub-assembly in a upper level assembly without affecting the sub-assembly file.  That only hides the component in the assembly where it was hidden.  As you can see in the screenshot below, I hid a mailbox that's part of a sub-assembly.  The sub-assembly is used twice in that assembly, and the other instance is unaffected.  Of course the hidden component will still show up in a BOM, but you didn't mention BOMs so I don't know if that's a concern for you.