The invention discloses a preparation method of a C5, C6 mesotherm isomerization catalyst, and belongs to the field of light alkane isomerization. The preparation method provided by the invention comprises steps that performing preliminary modification on a catalyst carrier prepared by hydrogenated mordenite/aluminium oxide through adopting a citric acid-methyl alcohol solution, then dipping in a platinum salt solution. By means of the operation, the conversion rate and selectivity of the catalyst can be improved efficiently, thereby obtaining the C5, C6 mesotherm isomerization catalyst with high performance.