Abstract: Objectives: Precise slant ionospheric delay reference is the key to rapid initialization of Global Navigation Satellite System (GNSS) Precise Point Positioning (PPP). The accuracy of regional slant ionospheric models (SIM) is closely related to modeling location and betweenstation distance. How the above factors influence SIM accuracy and PPP-RTK positioning is analyzed. Methods: Slant ionospheric delays were extracted using uncombined PPP-Ambiguity Resolution (AR), and the receiver code bias in them was canceled by making differences between satellites. The SIMs consisted of polynomial functions and residual grids and were constructed under different between-station distance conditions. Then we assessed accuracies of SIM and used them for PPP-RTK positioning to inspect improvement compared with PPP-AR. Results: The experiments were conducted in Europe and Yunnan Province respectively, which were very different in latitudes. During the magnetically quiet days, with an average between-station distance of 417 km in Europe, the external accuracy was better than 0.4 TECu overall and the PPP-RTK solutions reduced horizontal and vertical convergence time by 82% and 44% respectively. In Yunnan Province, China, only when the average between-station distance was less than 130 km could the external accuracies and the PPP-RTK enhancement approach those of Europe. In terms of between-station distance, the SIM accuracies showed slight differences when it expanded to 611 km in Europe during the magnetically quiet days, whereas an evident trend of decreasing model accuracy with increasing between-station distance occurred during the magnetic storm or in Yunnan Province. Conclusions: For regional ionospheric modeling in mid-to-high latitude regions such as Europe, the ionosphere is spatially smooth enough, and the between-station distance there can expand appropriately but not too much in case of PPP-RTK degradation during magnetic disturbance. While in low latitude regions such as Yunnan Province, due to the equatorial ionization anomaly, We do not recommend between-station distance to be larger than 100 km, otherwise the model accuracy can not meet the users’ demand.