var obj = document.getElementById(select_id);

for(i=0;i<obj.length;i++){

  if(obj[i].value == something)

    obj[i].selected = true;

}

<select id="test">

    <option selected value="0">- 请选择 -</option>

    <option value="1">关羽</option>

    <option value="2">赵云</option>

    <option value="3">马超</option>

    <option value="4">黄忠</option>

    <option value="5">张飞</option>

</select>

<input type="text" id="sel_text"/><input type='button' value='设置选项' onclick="fun()"/>

function fun(){

    var str = document.getElementById("sel_text").value;

    var obj = document.getElementById("test");

    for(i=0;i<obj.length;i++){

        if(obj[i].value==str)

            obj[i].selected = true;

    }

}