参考博客
1. 检查模型所用到的Shader是否添加到 Always Included Shaders,如下图

2. 打包的时候可以不添加后缀（添加MIME类型如图2）或者给添加的后缀配置MIME类型(如图3)

application/octet-stream：代表任意的二进制数据传输

3.设置目录浏览权限,默认是禁用的,因为加载ab包需要访问目录,如下图

4.添加HTTP响应标头：

<customHeaders>
	<add name="Access-Control-Allow-Credentials" value="true" />
    <add name="Access-Control-Allow-Headers" value="X-Requested-With,origin,content-type,accept" />
    <add name="Access-Control-Allow-Methods" value="GET, PUT,POST,DELETE,HEAD OPTIONS" />
    <add name="Access-Control-Allow-Origin" value="*" />
 </customHeaders>

结果如下图

5.在Player Setting中不要勾选Strip Engine Code,如下图

打包AB包的方式,网上很多,我就不记录了,以下是我从服务器加载的代码

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.Networking;

public class LoadAB : MonoBehaviour
{
    void Start()
    {
        StartCoroutine(InstantiateObject());
    }


    IEnumerator InstantiateObject()
    {
        string uri = @"http://localhost:8081/AssetBundles/ga";

        UnityWebRequest request = UnityWebRequestAssetBundle.GetAssetBundle(uri);
        yield return request.SendWebRequest();

        if (request.isHttpError || request.isNetworkError)
        {
            Debug.Log("错误:" + request.error);
        }

        else
        {
            AssetBundle ab = DownloadHandlerAssetBundle.GetContent(request);
            var game = ab.LoadAsset<GameObject>("Cube");

            Instantiate(game);

            Debug.Log("加载完成了");
        }
    }
}