<script type="text/javascript">
    function checkpassword() {
        var password = document.getElementById("pw").value;
        var repassword = document.getElementById("repw").value;

        if(password == repassword) {
            document.getElementById("tishi").innerHTML="<br><font color='green'>两次密码输入一致</font>";
            document.getElementById("submit").disabled = false;

        }else {
            document.getElementById("tishi").innerHTML="<br><font color='red'>两次输入密码不一致!</font>";
            document.getElementById("submit").disabled = true;
        }
    }
</script>

这段是JavaScript代码，假设两次密码不一致，则显示两次输入密码不一致

<input type="password" id="pw" name="passwd" placeholder="请输入密码" onkeyup="checkpassword()">
<input type="password" id="repw" name="repasswd" placeholder="请再次输入密码" onkeyup="checkpassword()">

<span id="tishi"></span>

效果如下