After I create a new Portal,

Question: I use "Service Portal Configuration/Branding Editor" to set new Logo and Title for this custom portal,

but for users accessing on new sp url, the default logo is still set in the header on public SP Login page.

Where can i set the new logo instead ?

Answer: Go to "Service Portal Configuration/Portals", and chose your custom portal,

On the root part, If "Login page" field is empty, ServiceNow use default settings.

Just reference 'login' page as your "Login page" in this field and it'll use your settings.