经过研究，以下是对我有效的方法：

<?php
$paragraphAfter= 1; //show box after selected paragraph
$content = apply_filters(\'the_content\', get_the_content());
$content = explode("</p>", $content);
for ($i = 0; $i <count($content); $i++) {
if ($i == $paragraphAfter) { ?>
<!-- START OF BLOCKQUOTE -->
stuff
<!-- END OF BLOCKQUOTE -->
<?php
}
echo $content[$i] . "</p>";
} ?>