The invention relates to a storage tank composite foundation variable stiffness adjustment optimization design method. The method comprises the following steps of calculating settlement of a compositefoundation, drawing a settlement contour map, and dividing a storage tank foundation into two parts including a settlement core region and a settlement non-core region according to settlement distribution characteristics of the storage tank composite foundation; according to the height, diameter and load of a storage tank and the strata distribution and physical-mechanical properties of the tankfoundation, increasing the stiffness of the settlement core region or reducing the stiffness of the settlement non-core region or increasing the stiffness of the settlement core region and reducing the stiffness of the settlement non-core region, thereby enabling a settlement difference of the settlement core region and the settlement non-core region to be smaller than a set value; and performingcheck calculation on overall bearing force of the composite foundation. The engineering investment of storage tank foundation treatment can be saved; the settlement of the storage tank foundation is effectively reduced, and especially non-uniform settlement is reduced; and a stress state of a storage tank structure can be improved.