when i open a website,i want to display popup window

I guess this should work.

Place a 'div' in your page as display hidden.
Something like:

<div id="divMain" style="position: absolute; display: None; z-index: 1001;"></div>

Now, on page, in script tag, just make the display as display for this div:

document.getElementById("divMain").style.display = "block";
// set other properties of div like position
// once some event happens, hide it again.

Two things to notice, you show it as the page load is complete. You keep the zIndex higher than normal page with position absolute.

Try!