搞出來了：
{if empty($eyou.field.xg_id_list)}
{eyou:relevarticle loop="3"}
<a href="{$field.arcurl}">{$field.title}</a><br>
{/eyou:relevarticle}
{else}
{eyou:arclist idlist='$eyou.field.xg_id_list' row='10' typeid="4"}
<ul>
<li><a href="{$field.arcurl}">{$field.title}</a></li>
<p>{$field.seo_description}</p>
</ul>
{/eyou:arclist}
{/if}