Is there any boot complete trigger event in windows phone 8 ? I have search a lot on google but I could not get any information about boot trigger event in windows phone 8 .
WP7/WP8 doesn't allow for full multitasking. As part of that you can't choose specific OS events to triggeryou code. There are fixed extensiblity points the OS exposes through various means you can tap into. For example, PeriodicBackgroundAgents starts up on a schedule of 30+ minutes for 10 seconds. IntensiveResourceBackgroundAgent starts up when the phone is connected to WiFi and Battery. However there's currently no OS extensiblity point for startup exposed to 3rd party apps.
If you'd like to see this feature please open up a new wpdev uservoice request with your usecase. Note, overall device health in terms of battery life, memory and processing capability is always a concern. So make your usecase shine.