The fictive temperature (Tf) is widely applied to understand the relaxation thermodynamics of a glass; however, its atomic structural origin is still unclear. Here, we report two novel AZnP3O9 glasses obtained by melting the composition identical single crystals. These glasses exhibit structural inheritance within 5 Å from the single crystal counterparts that is quantified by δ=nglass/ncry (0≤δ≤1, n is the number of pair correlation functions). Among the available glass‐formers, glassKZnP3O9 exhibits the highest structural inheritance (δ=1, nglass=8). More insightfully, a reverse correlation between δ and the relaxation thermodynamic parameters is observed in glassAZnP3O9, revealing for the first time the atomic structural origin of fictive temperature. A new evaluation method is provided to judge the glass stability and glass‐forming ability. As an example, the reported novel glass KZnP3O9 exhibits the highest δ (δ=1, nglass=8) that quantitatively identifies the short‐range order similarity to its crystalline counterpart. The inverse δ‐Tf correlation gives the direct evidence for the structural origin of fictive temperature revealing the relaxation thermodynamics.