An improvedmacroscopic-microscopic nuclear mass formula is presented in which shell and pairing effects are simultaneously evaluated by a procedure similar to Strutinsky method. The coefficients of the macroscopic-microscopic mass formula have been adjusted on 2267 experimental atomic masses extracted from the atomic mass evaluation of 2012 (AME2012). Same as inthe Weizsäcker-Skyrme (WS) model, the influence of the nuclear deformation on the macroscopic energy as well as the mirror nuclei constraint istaken into account, and for the sake of the consistency of the model parameters between the macroscopic and the microscopic parts we approximate the isospin-dependent component of the macroscopic energy to the depth of the Woods-Saxon potential. As a result, the root-mean square (rms) deviation with respect to 2267 measured nuclear masses is 0.493MeV. Then,based on the fitted formula we predict the remaining 988 nuclei from the AME2012 for which the masses are still unknown or not well-known, and calculate the α-decay energies of seven chains in the superheavy nuclei region with Z=117 and 118.