We investigated the impact of substituting B2O3 for P2O5 on a refractive index and optical absorption of pyro SnO−P2O5−B2O3 glasses. To date, we have reported that thermal properties and glass structure have a characteristic composition dependence when B2O3 replaces P2O5. In this paper, we report that optical properties such as optical absorption and refractive index also have a characteristic composition dependence. We observed a consistent increase in the refractive index with increasing B2O3 substitution, categorized into three regions, which is consistent with the trend in density. In addition, we observed that the optical absorption edge of the glass is shifted to a longer wavelength with increasing B2O3 concentration. To gain further insight into the optical properties of the glasses, we utilized Kramers–Kronig analysis to experimentally correlate the observed changes in refractive index and optical absorption. These changes in density and refractive index were predicted by elucidating the short-range structures of the glass, which were quantitatively provided by determining the proportions of phosphate and borate units. This information was then used to calculate the ion packing ratio.