js必选在</form>外面才有用

 <div class="reg_checkboxline pos">
            
      <span class="z"><input id="checked" type="checkbox" onclick="confirmBox()"/></i></span>
      <div class="Validform_checktip"></div>
      <p>我已阅读并接受(必选) <a href="javascript:void(0)" class="click_pop">《用户注册协议说明》</a></p>
                    
      </div>
                <div class="dl-btn">
                <input class="lang-btn off log-btn" type='submit' id="submitBtn" name='Submit' value='马上注册' disabled="disabled"> 
                </div>
            </form>
<script type="text/javascript">
function confirmBox(){
  if(document.getElementById("checked").checked==true)
  {
      document.getElementById("submitBtn").disabled="";
  }else{
      document.getElementById("submitBtn").disabled="disabled";
  }
}
</script>

需要注意修改的地方
<input id="checked" type="checkbox"/>
<input class="lang-btn off log-btn" type='submit' id="submitBtn" name='Submit' value='马上注册' disabled="disabled">
disabled="disabled"