你应该使用递归:

public void SomeMethod() {
     // here you get your `list`
     var tree = GetTree(list, 0);
}

public List<Tree> GetTree(List<Personal> list, int parent) {
    return list.Where(x => x.ParentId == parent).Select(x => new Tree {
        Id = x.Id,
        Name = x.Name,
        List = GetTree(list, x.Id)
   }).ToList();
}