In harsh offshore environments, large-area sintered nano-copper (Cu) interconnections, which serve as die attachment material or thermal interface material (TIM), are prone to degradation from hydrogen sulfide (H2S) corrosion. This study introduced a film-forming technique based on atmospheric pressure plasma jet (APPJ) to improve the corrosion resistance of large-area sintered nanoCu joint. The corrosion protection mechanism against H2S-containing atmospheric corrosion was investigated using both experimental methods and density functional theory (DFT) simulations. The key findings were as follows: (1) The deposition film, primarily composed of a Si-O3 network, effectively protected sintered Cu plate from corrosion by H2S gas, and maintaining the mechanical performance of sintered Cu joint after 384 h of H2S testing. (2) The dissociation products of the APPJ-treated precursor hexamethyldisiloxane (HMDSO), −OSiCH3 and −OSi(CH3)3, formed stable chemical bonds on the sintered nanoCu surface, resulting in the formation of −OSiCH3(O-CH3)2 fragments. (3) The −OSiCH3(O-CH3)2 fragments were unreactive toward to corrosion agents such as H2S, O2, and H2O, and also serving as a barrier to block their access to the sintered nanoCu surface. This study provided a comprehensive understanding of the corrosion protection mechanism of sintered nanoCu using APPJ-deposited films, offering valuable insights for improving the reliability of power electronics.