The quest for novel nonlinear optical (NLO) polymorphs is pivotal for advancing laser technology and frequency conversion applications. We present a detailed study on the synthesis and properties of the Fdd2 polymorph of C12H8N2O4Zn, which exhibits an enhanced second harmonic generation (SHG) effect, 0.3 times that of KDP, nearly 8-fold higher than the P43212 phase. This improvement is attributed to an increased [ZnN2O4] polyhedral stacking angle and structural distortion in the Fdd2 phase. Our findings underscore the importance of polymorphs in optimizing NLO properties and position the Fdd2 phase as a promising candidate for ultraviolet NLO applications.