The inverted p-i-n structured perovskite solar cells (PSCs) using the charge-selective contacts with less sensitivity to ambient condition is regarded as a viable route to fabricate high-performance PSCs with better environmental stability, and nickel oxide (NiO_x) is the most frequently used hole-selective contact at the illumination side due to its high carrier mobility, low cost, and high transparency. However, the lifetime of NiO_x-based inverted PSCs is still relatively short (around 1000 hours), and further enhancement of their long-term operational stability is largely limited by the light-induced degradation at the NiO_x-perovskite heterojunction. In this regard, a simple strategy based on construction of a stable buffer layer that can synergistically eliminate the light-induced degradation and minimize the charge recombination loss in NiO_x-based inverted PSCs is highly desirable, and the development of in-situ characterization techniques are urgently needed to identify the degradation products and further clarify the degradation mechanism. In this work, we used time-resolved quadrupole mass spectrometry (MS) technique to reveal the degradation mechanism of NiO_x-FAMAPbI₃  (formamidinium-methylammonium iodide) perovskite heterojunction under operational condition: 1) generation of iodine vapor and free proton via the oxidation and deprotonation reactions; 2) formation of volatile products under elevated temperature, including hydrogen cyanide, methyliodide, and ammonia; 3) formation of condensation product, N-methyl formamidine, with the increased vapor pressures of dissociated FA and MA molecules. Inspired by the aprotic nature, good phototability, suitable size of trimethylsulfonium (TMS⁺), and the high oxidation potential of Br^-, we introduced a novel TMSBr buffer layer between NiO_x and perovskite to eliminate these multi-step photochemical reactions. Consequently, the TMSBr-stabilized inverted PSCs (1.53-eV bandgap) exhibited a promising efficiency of 22.1% with an open-circuit voltage of 1.18 V, and could retain approximately 82.8% of the initial efficiency after 2000-hour operation under AM1.5G light illumination, which translates into an estimated T80 lifetime of 2310 hours (the time over which the efficiency reduces to 80% of its initial value), which is among the highest operational lifetimes reported for NiO_x-based PSCs.