The influence of an external magnetic field on successive phase transitions, specifically of isostructural mode, taking place in smectic A liquid crystals with positive molecular anisotropy of magnetic susceptibility is studied on the basis of a broaden version of the mean-field Landau-de Gennes model. Topological classification of bifurcation sets (separatrices) and phase diagrams of the model are performed. It is shown that there are two critical values of the field : at the smaller value the isostructural phase transition of nematic-paranematic (N-pN) type cannot be realized and at the greater one the isostructural phase transition of smectic-smectic (Sm-Sm) type is absolutely suppressed. It is found a new-kind of critical point in which the Sm-Sm critical end point, the triple Sm-Sm-N point and tricritical Sm-N point merge.