As a progressive neurodegenerative disease, Alzheimer's disease (AD) is characterized by the accumulation and misfolding of amyloid polypeptides (Aβ) which result in irreversible brain damages, cognitive decline and behavioral impairment. Photodynamic therapy (PDT) has received much attention as a potential approach for AD treatment. In this study, we investigated a novel multifunctional theranostic photosensitizer, copper cysteine (Cu-Cy). By performing PEG modification on nanoparticles’ surface, the particle size and cytotoxicity were significantly reduced. Under UV irradiation, Cu-Cy-PEG can produce reactive oxygen species (ROS) and could degrade Aβ(1-40) aggregates efficiently. This is the first case for Cu-Cy-PEG being used for degrading Aβ aggregates in vitro. The macroscopic amyloid aggregates were photodegraded by Cu-Cy-PEG into amorphous particles. This study firstly proved that Cu-Cy-PEG may serve as a potential treatment in AD therapy.