after I use: JSON.stringify and JSON.parse to execute the returned object, I can add properties to this object, but I'm still not clear about the specific reason.
the latest modification:
find the API of Sequelize and found that there is a setDataValue method that can be used to query the returned object settings. Just change the code of the original problem to the following:

for (let i = 0; i < orderList.length; iPP) {
    let userInfo = await wdUser.findById(orderList[i].user_id)
    orderList[i].setDataValue('user_name',userInfo.user_name)
    orderList[i].setDataValue('user_headurl',userInfo.user_headurl)
}

API link: https://itbilu.com/nodejs/npm.

add a raw: true condition to the query, and you can assign the value directly.

mongoose overrides the toJSON method. Properties you add later will not be returned in the toJSON method. That is to say,

you need to convert document object to javascript object
link