The intense field-ionization behavior of linear homonuclear polyatomic molecular ions has been numerically investigated for different molecular configurations. The enhanced ionization effect of molecular ions is found as a general feature in a critical bond length range, but the enhancement degree of ionization is bond-selective. The more symmetric the molecular ion stretches, the more enhanced the ionization of the molecular ion. It implies that the symmetric fragmentation of linear molecular ions is most probable at moderate laser intensity. These results are explained in terms of the field-induced over-barrier ionization mechanism.