The aim of this article is to provide a systematic method to perform numerical and experimental evaluation on the optical properties of Al-doped ZnO nano thin films. Some different doping density samples are deposited for testing the transmittance. The results show that the transmittance of Al-doped ZnO has a nonlinear relation with the optical constants and Al-doped quantity. The optical band gap of the Al-doped ZnO decreases with the Al doping quantity increasing. Meanwhile, the Maxwell-Garnett (MG) theory is used to investigate the optical properties of Al-doped ZnO nano thin films in visible range. The comparison can illustrate the validity of both test and design method. It implies a potential design and evaluation method for developing a new type of ceramic nano thin film in engineering.