from urllib.parse import quote
@file.route('/download', methods=["POST"])
def download_file():
  filename='xx' #文件名
  filepath='xx/xx' #文件路径
  res = make_response(send_file(filepath))
  #自定义的一个header，方便前端取到名字
  res.headers['filename'] = quote(filename.encode('utf-8'))
  return res
javascript——以async异步fetch为例：

async function download() {
  const res = await fetch(`http://xxx/file/download`, {
  method: "POST",
  body: JSON.stringify({}), //body里面是要发送的数据
  headers: { "Content-Type": "application/json" },
  responseType: 'blob'
 })

 if (res.ok) {
  const blData = await res.blob() //拿到blob数据
  const urlObjData = window.URL.createObjectURL(new Blob([blData])) //创建url对象
  
  //获取文件 进行下转码
  const fileName = decodeURI(fileNameres.headers.get('filename'))
  
  //创建a标签 点击a标签 达到下载目的
  const link = document.createElement('a')
  link.href = urlObjData
  link.download = fileName //下载文件的名字
  document.body.appendChild(link)
  link.click()
  
  document.body.removeChild(link)
  window.URL.revokeObjectURL(urlObjData);
  
  //展示图片
  //xxx.src=urlObjData
 }
}

@app.route('/downloadfile/', methods=['GET', 'POST'])
def downloadfile():
  if request.method == 'GET':
    fullfilename = request.args.get('filename')
　　　　# fullfilename = '/root/allfile/123.txt'
    fullfilenamelist = fullfilename.split('/')
    filename = fullfilenamelist[-1]
    filepath = fullfilename.replace('/%s'%filename, '')
    #普通下载
    # response = make_response(send_from_directory(filepath, filename, as_attachment=True))
    # response.headers["Content-Disposition"] = "attachment; filename={}".format(filepath.encode().decode('latin-1'))
    #return send_from_directory(filepath, filename, as_attachment=True)
    #流式读取
    def send_file():
      store_path = fullfilename
      with open(store_path, 'rb') as targetfile:
        while 1:
          data = targetfile.read(20 * 1024 * 1024)  # 每次读取20M
          if not data:
            break
          yield data

    response = Response(send_file(), content_type='application/octet-stream')
    response.headers["Content-disposition"] = 'attachment; filename=%s' % filename  # 如果不加上这行代码，导致下图的问题
    return response