判断为手机端：

var sUserAgent = navigator.userAgent.toLowerCase();
var bIsIpad = sUserAgent.match(/ipad/i) == "ipad";
var bIsIphoneOs = sUserAgent.match(/iphone os/i) == "iphone os";
var bIsMidp = sUserAgent.match(/midp/i) == "midp";
var bIsUc7 = sUserAgent.match(/rv:1.2.3.4/i) == "rv:1.2.3.4";
var bIsUc = sUserAgent.match(/ucweb/i) == "ucweb";
var bIsAndroid = sUserAgent.match(/android/i) == "android";
var bIsCE = sUserAgent.match(/windows ce/i) == "windows ce";
var bIsWM = sUserAgent.match(/windows mobile/i) == "windows mobile";
if (bIsIpad || bIsIphoneOs || bIsMidp || bIsUc7 || bIsUc || bIsAndroid || bIsCE || bIsWM) {
        // 手机端         
 } else {
        // pc端
  }

<script type="text/javascript">
// 判断为移动端
    var sUserAgent = navigator.userAgent.toLowerCase();
    if (/ipad|iphone|midp|rv:1.2.3.4|ucweb|android|windows ce|windows mobile/.test(sUserAgent)) {
        //移动端    
        window.location = "m/index.html";
    }
</script>

这两个都可以用，希望对大家有帮助！