A high-temperature and high-pressure corrosion-resistant process heat exchanger for a nuclear hydrogen production system decomposes sulfite (SO3) using heat from a high-temperature gas-cooled reactor to thereby produce sulfide (SO2) and oxygen (O2). The process heat exchanger comprises second and third system coolant channels, each of which is defined by a heat transmission fin, which is bent in a quadrilateral shape, and heat transmission plates, and has increased corrosion resistance thanks to ion-beam coating and ion-beam mixing using a material having high corrosion resistance. The third system coolant channel includes reaction catalysts for SO3 decomposition, and is made of a super alloy. Thus, a system differential pressure between the second and third system coolant channels can be greatly maintained at a high temperature of 900° C. or higher.