Natural casing bundling problem is a multi-objective combinatorial optimization problem. To approach the problem，the bundles are categorized depending on the length of their shortest strand，and the categories are put in sequence from high quality to low quality consistent with the length. The multilevel optimization method is adopted to figure out the maximal number of bundles in each category ranging from high to low. Thus the global optimization is achieved through optimizing each individual category. At last，a complete mathematical model is established. As an application, the bundling scheme is given out according to the actual data，and the final answer of 187 bundles is obtained