Recently, photodynamic therapy (PDT) has been extensively applied in clinical and coadjuvant treatment of various kinds of tumors. However, the photosensitizer (PS) of PDT still lack of high production of singlet oxygen (1O2), low cytotoxicity and high biocompatibility. Herein, we propose a facile method for establishing a new core-shell structured Sn nanocluster@carbon dots (CDs) PS. Firstly, Sn4+@S-CDs complex is synthesized using the sulfur-doped CDs (S-CDs) and SnCl4 as raw materials, and subsequently the new PS (Sn nanocluster@CDs) is obtained after vaporization of Sn4+@S-CDs solution. Remarkably, the obtained Sn nanocluster@CDs show an enhanced fluorescence as well as a higher 1O2 quantum yield (QY) than S-CDs. The high 1O2 QY (58.3%) irradiated by the LED light (400-700nm, 40mW/cm2), induce the reduction of 4T1 cancer cells viability by 25%. More intriguingly, no visible damage happens to healthy cells, with little impact on liver tissue due to renal excretion, both in vitro and in vivo experiments demonstrate that Sn nanocluster@CDs may become a promising PS, owning a high potential for application in PDT.