Total loss test of the high-reflective (HR) film coated on super-smooth silica substrate by dual ion beam sputtering (DIBS) is based on the well-established cavity ring-down technique. Scattering and transmittance are tested by integral scattering and transmittance measuring apparatus, after which absorption is calculated. At 632.8 nm wavelength, the magnitude and distribution of thin film loss are researched for both s- and p-polarization, and the reflectivities are 0.99986, 0.99997, and 0.99962, respectively. Based on the analysis, the tested scattering is less than its real value.