There is something wrong with you just writing it like this
It should be

 父组件有一个方法 methods: { alertFun () { alert(2) } }

What should be passed here is a function name of the parent component method instead of writing alert(2) directly

This should be the problem

The for in object loop obtains the index, not value, so branch.id cannot be obtained, it can be changed to for of

The following is a sub-component @change='showChange' is a sub-component event
The following template is registered as an order-type component

The following are sub-component methods:

showChange(event) { for (let branch of this.branchList) { if (branch['id'] === event.target.value) { /注意此行的修改/ this.$emit('parentChage',branch['prefix']); } } 以下是父组件  但alert(2) 并未执行