Abstract: The passive autocatalytic recombiner (PAR) is the main measure of hydrogen elimination in nuclear power plant. In severe accident, the presence of some impurities (e.g. I₂, CsI, CO etc.) produced by fission reaction can have an adverse impact on catalyst functioning, and the relevant poison mechanism study is urgently needed to ensure reliability of PAR’s performance. According to results of relevant tests, the mechanism study was conducted from distribution structure of Pt and Pd atoms and active surface of catalyst. It is concluded that the distribution structure of metal atoms in catalyst and the real concentration of poisons flowed in PAR are key factors in poisoning effect.