template\default\forum\post_editor_option.htm
<p class="mbn"><input type="checkbox" name="htmlon" id="htmlon" class="pc" value="1" $htmloncheck /><label for="htmlon">{lang post_html}</label></p>
改成
<p class="mbn"><input type="checkbox" name="htmlon" id="htmlon" class="pc" value="1" $htmloncheck checked="checked" /><label for="htmlon">{lang post_html}</label></p>