代码：

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.SceneManagement;
public class NewBehaviourScript : MonoBehaviour
{
    float f;
    void Start()
    {
        f = 0;
    }
    public void LoadNextScene()
    {
        StartCoroutine(LoadAsyneScene());        
    }
    IEnumerator LoadAsyneScene()
    {
        //把想要加载的场景id赋值给i
        int i = 1;
        //异步加载场景
        AsyncOperation asyneLoad = SceneManager.LoadSceneAsync(i);
        asyneLoad.allowSceneActivation = false;
        while (!asyneLoad.isDone)
        {
            //设置加载下一场景的条件：一秒后加载
            if (f<1)
            {
                f += Time.deltaTime;
                Debug.Log(f);
            }
            else
            {
                asyneLoad.allowSceneActivation = true;
            }
            yield return null;
        }
    }
}

以上是最简单的一个异步加载场景：一秒以后加载下一场景。
这里是一位作者写的比较详细的异步加载