Menu Fix in WordPress

Solution:

if your menu structure is like below:

<ul class="parent-menu">
    <li><a href="#">Menu1</a></li>
    <li><a href="#">Menu1</a>
        <ul class="sub-menu">
            <li><a href="#">Sub Menu1</a></li>
            <li><a href="#">Sub Menu1</a></li>
            <li><a href="#">Sub Menu1</a></li>
        </ul>
    </li>
    <li><a href="#">Menu1</a></li>
</ul>

Then you can use css like this:

ul.parent-menu li {
    position: relative;
}

ul.parent-menu li ul.sub-menu {
    display: none;
    position: absolute;
    left: 0;
    top: 100%;
}

ul.parent-menu li:hover > ul.sub-menu {
    display: block;
}