To provide a method for preparing a stable PROX (preferential oxidation) reaction catalyst.

This method for preparing the stable PROX reaction catalyst comprises: a step (a) of preparing a platinum catalyst supported on alumina; a step (b) of calcining the catalyst from the step (a) at 300-600°C for 2-4 hours; a step (c) of bringing the catalyst obtained from the step (b) into contact with hydrogen at 300-600°C for 2-4 hours to reduce the catalyst; a step (d) of dropping the temperature in the reduced catalyst reduced from the step (c) to <100°C, and bringing the resulting catalyst into contact with a predetermined solvent and/or gas while raising the temperature of the resulting catalyst to 300-600°C. The catalyst has a Pt-Al arrangement, the content of Pt being 0.5-7 wt% and has an overall shape of an oval.