$(function () {
    var nav = $("#w_grid-1577200068619"); //寰楀埌瀵艰埅瀵硅薄  
    var win = $(window); //寰楀埌绐楀彛瀵硅薄  
    var sc = $(document); //寰楀埌document鏂囨。瀵硅薄銆�  
    win.scroll(function () {
        if (sc.scrollTop() >= 200) {
            nav.addClass("fixednav");
        } else {
            nav.removeClass("fixednav");
        }
    });
});