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<div class="outer">
    <div class="inner">
        <p>第一行文字
            <br/>第二行文字
            <br/>第三行文字
            <br/>
        </p>
    </div>
</div>
<div class="outer">
    <div class="inner">
        <p>第一行文字</p>
    </div>
</div>

* {
    margin: 0;
    padding: 0;
}
.outer {
    background:#eee;
    height: 175px;
    width: 275px;
    position: relative;
    display: table-cell;
    vertical-align: middle;
    padding: 5px;
    border: 1px solid #ccc;
}
.inner {
    position:static;
    *position:absolute;
    top: 50%;
    width:100%;
    float:left;
}
p {
    background:#fff;
    position: relative;
    top: -50%;
    padding:10px;
    color: #222;
    font-size: 18px;
}