Fluorophosphate glass is the core of laser materials and also has an important reference value for an optical lens. According to Kirchhoff’s law, this paper focuses on the optical loss of incident light entering fluorophosphate glass. The transmittance and refractive index of optical glass were measured by an ultraviolet-visible spectrophotometer (Uv) and V-prism tester. With the sample thickness increasing from 10 mm to 50 mm, the transmittance of grade 4 fluorophosphate glass decreases from 85.32 % to 76.91 %, and the scattering rate increases from 8.4704 % to 17.0591 %. The microstructure and internal images of macroscopic defects were obtained by projective optical microscopy. In addition, an electron probe (EPMA) was used to analyze the defective components and identify the main components of the defects. The results show that the high content of P/Al/O element is the main cause of optical loss.