The exsolution of transition metals in perovskite oxides has been actively researched for intelligent catalyst design in energy-related applications. To significantly increase the amount of exsolved particles, the complete phase reconstruction from simple perovskite to Ruddlesden-Popper (R-P) perovskite is greatly desirable. However, a comprehensive understanding of key parameters affecting the phase reconstruction to R-P perovskite is still unexplored. Herein, the oxygen vacancy formation energies (Evf-O) from PrO and TO2 in Pr0.5(Ba/Sr)0.5TO3-δ (T = Mn, Fe, Co, and Ni) are proposed as the important factor in determining the type of phase reconstruction in perovskites. Furthermore, using in-situ temperature & environment-controlled X-ray diffraction measurements, we mapped out the phase diagram and found the optimum ‘x’ range required for the complete phase reconstruction to R-P perovskite (x ≥ 0.3) in Pr0.5Ba0.5-xSrxFeO3-δ (PBSF) system. Among PBSF, the (Pr0.5Ba0.2Sr0.3)2FeO4+δ – Fe metal (R-PBSF30) has the smallest size of exsolved Fe metal particles when the phase reconstruction occurs from simple perovskite under reducing condition. The exsolved nano-Fe metal particles exhibited high particle density and are well-distributed on the perovskite surface, showing great catalytic activity in fuel cell mode (1.23 W cm-2 at 800 oC) and high syngas production by co-electrolysis of CO2 and H2O (–1.62 A cm-2 at 1.5 V, 800 oC).