Chalcogenide glass has become one of the essential IR lens materials in passively athermalized long-wave IR devices. However, that there is no multispectral chalcogenide glass capable of large-size fabrication raises challenges to the development and popularization of multispectral imaging systems combining visible, near-IR, and mid-IR. In this work, we developed a novel chalcogenide glass capable of a record-big (Ø120 mm) fabrication through the compositional optimization of GeS2-Ga2S3-CsCl glass with introduction of Sb2S3. Its transmission window is characterized as ranging from 0.51 to 11.2 µm, which means it could be employed as a multispectral lens transmitting visible and IR signals in a co-aperture IR optical system. In addition, a method of three-stage thermal analysis is proposed to evaluate the glass-forming ability of chalcogenide glass through simulating the melt-quenching process of chalcogenide melt in a vacuum-sealed silica ampoule. This work not only shows an innovative multispectral chalcogenide glass with promising applications but also introduces a simple and convenient technique for screening chalcogenide glass with ultrahigh glass-forming ability capable of large-size fabrication.