<p>The isospin asymmetry and quadrupole deformation value of drip-line nuclei are investigated using the Weizsäcker–Skyrme nuclear mass formula. We observe that for heavy nuclei at the neutron drip line, the Coulomb energy heightened by an augmented charge could not be mitigated completely by symmetry energy because of isospin asymmetry saturation but is resisted complementally by strong nuclear deformation. The positions of saltation for the difference in proton numbers between two neighboring nuclei at the neutron drip line, and the isospin asymmetry of the neutron drip-line nucleus as a function of the neutron number distinctly correspond to the known magic numbers, which can serve as a reference to verify the undetermined neutron magic number. Through fitting of the binding energy difference between mirror nuclei (BEDbMN), a set of Coulomb energy coefficients with greater accuracy is obtained. A high-precision description of the BEDbMN is useful for accurately determining the experimentally unknown mass of the nucleus close to the proton drip line if the mass of its mirror nucleus is measured experimentally.</p>