Quantitative composition‐structure‐property (C–S–P) relationship analysis is a promising approach to correlate structural features with properties of glasses that have complex structures and lack long‐range order. In this study, the structure and spectroscopic properties of two typical P2O5‐MO‐K2O‐Al2O3 (M = Mg, Ba) quaternary phosphate laser glasses doped with Nd3+ ions are calculated and predicted. By using the nearest‐neighboring congruently melting compounds in the phase diagram as “component and structural motifs”, correlations with properties and insights on glass behaviors can be obtained. Two key physical properties and five spectroscopic properties are predicted and verified with an error of less than 5% and 10%, respectively. Furthermore, two C–S–P databases with more than 1000 glass compositions are established for designing and optimizing new laser glasses. The results show that this approach not only gets rid of the traditional qualitative research method which relies on empirical formulas, but also promotes the quantitative calculation and predictable design of glass research.