I search the document but I don't find anything about ACL implementation in cakephp 3. How can I implement authorization with ACL in cakephp 3?
ACL is not built into CakePHP 3 as it was in CakePHP 2. It is now available as a separate plugin.
Quote from http://book.cakephp.org/3.0/en/appendices/3-0-migration-guide.html
ACL related classes were moved to a separate plugin. Password hashers, Authentication and Authorization providers where moved to the \Cake\Auth namespace. You are required to move your providers and hashers to the App\Auth namespace as well.
You can find the plugin at https://github.com/cakephp/acl, but note that it's not yet stable.