SOURCE

function code() {
  if ($('#qrtext').val().length > 353) {
    alert("有点太长了");
  } else {
    $('#qrcodeCanvas').html(""); 
     qrCode('qrcodeCanvas',  $('#qrtext').val(), 260);
  }
}

qrCode('qrcodeCanvas', "http://jsrun.net",260);
 

<div class="container">
<input id="qrtext" type="input" value="" placeholder="请输入文本" />
<button onclick="code()">
  生成
</button>
  <div class="des">暂不支持中文</div>
<div id="qrcodeCanvas">
</div>
  </div>

input {
  width: 299px;
  height: 25px;
  padding: 10px;
  border-radius: 25px 0px 0px 25px;
  outline: none;
  border: 1px solid #ccc;
}

.des{
  font-size:12px;
  margin:auto;
  width:100px;
  margin-top:15px; 
  color:#777;
}

button {
  height: 47px;
  margin-left: -9px;
  border-radius: 0px 25px 25px 0px;
  outline: none;
  width: 80px;
  border: 1px solid #217dcc;
  background: #3ca5ff;
  color: #FFF;
  font-size: 14px;
}
.container{
  margin: auto;
  width: 401px;
  margin-top: 100px;
}
#qrcodeCanvas {
  margin: auto;
  width: 256px;
  margin-top: 100px;
}