A method of preparing a supported catalyst includes dissolving a cation exchange polymer in alcohol to prepare a solution containing cation exchange polymer; mixing the cation exchange polymer containing solution with a catalytic metal precursor or a solution containing catalytic metal precursor; heating the mixture after adjusting its pH to a predetermined range; adding a reducing agent to the resultant and stirring the solution to reduce the catalytic metal precursor; mixing the resultant with a catalyst support; adding a precipitating agent to the resultant to form precipitates; and filtering and drying the precipitates. The method of preparing a supported catalyst can provide a highly dispersed supported catalyst containing catalytic metal particles with a reduced average size regardless of the type of catalyst support, which provides better catalytic activity than conventional catalysts at the same loading amount of catalytic metal.