Nyböite has been found in several sites of ultra-high pressure (UHP) eclogite in the world, but the p-T range of its stability is still controversial. In this study, the authors conducted a set of thermodynamic modelling in the system of Na₂O-CaO-K₂O-FeO-MgO-Al₂O₃-SiO₂-H₂O-TiO₂-Fe₂O₃ (NCKFMASHTO) to constrain the stability of nyböite, on the basis of the Jianchang eclogite (Sulu UHP metamorphic belt, northern Jiangsu Province). Modelling results reproduce the observed mineral assemblage and show that nyböite is only stable at t<500℃, p<1.20 GPa, and geothermal gradient of 12~17℃/km. The modeled composition of nyböite is broadly consistent with measured data. The formation of nyböite in eclogite is mainly controlled by X_Na and X_Al of the bulk composition, the condition of X_Na=0.58~0.65 and X_Al<0.15 is necessary for the formation of nyböite. The nyböite investigated here is characterized by enrichment of F (1.88%~2.54%), while F was not taken into account in the modelling for the lack of F-bearing amphibole thermodynamic model hitherto. However, the substitution F for OH seems to significantly affect the stability of nyböite. The occurrence of nyböite in eclogite indicates that the bulk composition is rich in Na and Al, possibly corresponding to an alkali basalt protolith, and the host rock underwent low temperature and intermediate to high pressure metamorphism.