function loading(p_value,str)
{
if (p_value)
{
if (!document.getElementById("load_area"))
{
var para1 = document.createElement("span");
var node=document.createTextNode(str);
para1.appendChild(node);
var para=document.createElement("div");
para.id="load_area";
var top=document.body.scrollTop+document.documentElement.scrollTop;//获取页面中的实际高度
top_position = top+157 + "px";
para.style.top =top_position;
icon.style.cssText += ";vertical-align:middle;padding-right:4px;margin-top:-2px;"
para.style.cssText +=
";position:absolute;left:50%;width:140px;margin-left:-70px;height:50px;line-height:50px;font-size:18px;text-overflow:ellipsis;overflow:hidden; white-space:nowrap;text-align:center;background-color: #000;border-radius:2px;-webkit-box-shadow:0 2px 2px rgba(0,0,0,0.5);color:#eee;";

var icon = new Image();
icon.src="http://www.mamicode.com/images/loading1.gif";//这里用实际路径替换
document.body.appendChild(para);

para.appendChild(icon);

para.appendChild(para1);

}
else
{
document.getElementById("load_area").style.display="block";
}
}
else
{
document.getElementById("load_area").style.display="none";
}
}

调用方法：loading(true,"加载中...");  loading(false)

模拟实现ajax加载框