A supported noble metal catalyst and a process for preparing the same in situ are provided. Hexamethylenetetramine, a soluble divalent metal salt solution, a Al2O3 carrier and a soluble noble metal salt solution, are mixed and crystallized, in which the hexamethylenetetramine acts as both a precipitating agent for producing hydrotalcite and a reducing agent for the noble metal precursor, and a supported catalyst Me-LDHs-Al2O3 containing an elementary substance of a noble metal is prepared by a one-step reaction. During the growth of the hydrotalcite, Al3+ on the surface layer of the Al2O3 carrier is directly used as the trivalent metal ion to form the slab structure of the hydrotalcite, and the hydrotalcite is grown in situ on the surface of the alumina carrier. The noble metal element particle in the catalyst has a particle size of 10 to 60 nm, and has an even and stable dispersion on or between slabs of the hydrotalcite. The calcination and reduction processes in the traditional production method are no longer required, and the reduced noble metal catalyst is obtained by a single step of a crystallization process, while the utilization ratio of the noble metal is high.