Disclosed are a hydrothermally stable porous molecular sieve catalyst and a preparation method thereof. The catalyst consists of a product obtained by the evaporation of water from a raw material mixture comprising a molecular sieve having a framework of Si-OH-Al-, a water- insoluble metal salt and a phosphate compound. The catalyst maintains its physical and chemical stabilities even in an atmosphere of high temperature and humidity. Accordingly, the catalyst shows excellent catalytic activity even when it is used in a severe process environment of high temperature and humidity in heterogeneous catalytic reactions, such as various oxidation/reduction reactions, including catalytic cracking reactions, isomerization reactions, alkylation reactions and esterification reactions.