停止表单提交，然后从模式框触发

document.getElementById("submitBtn").addEventListener("click", function(event) {
  event.preventDefault();
});

<button id="modalBtn">打开模态框</button>

<div id="myModal" class="modal">
  <div class="modal-content">
    <span class="close">&times;</span>
    <p>这是一个模态框。</p>
  </div>
</div>

<script>
// 获取模态框元素
var modal = document.getElementById("myModal");

// 获取触发模态框的按钮元素
var btn = document.getElementById("modalBtn");

// 获取关闭模态框的元素
var closeBtn = document.getElementsByClassName("close")[0];

// 点击按钮打开模态框
btn.onclick = function() {
  modal.style.display = "block";
}

// 点击关闭按钮关闭模态框
closeBtn.onclick = function() {
  modal.style.display = "none";
}

// 在用户点击模态框外部区域时关闭模态框
window.onclick = function(event) {
  if (event.target == modal) {
    modal.style.display = "none";
  }
}
</script>