Abstract: A photosensitizer and derivative, application thereof. The photosensitizer has the structure of general formula I, wherein X is S or Se, Y is organic or inorganic ion, R1 and R2 are independently selected from H, alkyl, alkoxy, alkyl amido, alkyl azide and the like; R3 is selected from H, alkyl, alkoxy, amino sulfonyl, hydroxyl, carboxyl and the like, and L1 is a linker selected from —(CH2)n1— or —(CH2CH2O)n2—. The derivatives are molecular medicines with drug molecules of anticancer and chemotherapy or tumor targeting function connected to the said photosensitizer. The photosensitizer has excellent near infrared characteristics and low dark toxicity and is used in the field of photodynamic tumor therapy. The introduction of benzophenothiazine or benzophenoselenazine into derivatives with tumor-targeting function could improve the specific uptake of photosensitizer in tumor tissues.