As one of the three key components of photodynamic therapy (PDT), photosensitizers (PSs) greatly influence the photodynamic e±ciency in the treatment of tumors. Photosensitizers with tetrapyrrole structure, such as porphyrins, chlorins and phthalocyanines, have been extensively investigated for PDT and some of them have already received clinical approval. However, only a few of porphyrin-based photosensitizers are available for clinical applications, and PDT has not received wide recognition in clinical practice. In this regard, PSs remain a limiting factor. Our research focuses on the rational design of new PSs. Photocyanine, a Zinc (II) phthalocyanine (ZnPc) type photosensitizer with low dark toxicity and high single oxygen quantum yield, is one of the promising PSs candidates and currently being tested in clinical trials. Here, we present an overview on the development of Photocyanine, including its design, synthesis, purification, characterization and preclinical studies, wishing to contribute to the research of more promising PSs.