What is the shortcut key to open debug tab in windchill ?

Not sure what you mean by the "debug tab" in Windchill. However if you are referring to the web browser debugging tools then that is browser dependent. In Chrome F12 key will open this window/tab. If you are referring the the jsDebug=1 appended to Windchill urls then see this:

http://support.ptc.com/help//wnc/r12.1.0.0/en/index.html#page/Windchill_Help_Center/customization/WCCG_UICust_CustHTMLClients_ToolsOview_EnablejsDebug.html