<select id="mainoption" name="mainoption">
    <option th:each=" mainoption : ${ mainoptionList } "
        th:text=" ${ mainoption } "></option>
</select>
<select id="suboption" name="suboption">
    <option th:each=" suboption : ${ suboptionList } "
        th:text=" ${ suboption.suboption } "></option>
</select>

$('#mainoption').on('change', function(){
});