In order to inhibit the Gd/Ba substitution in the growth process effectively, a series of single domain GdBCO bulk superconductors with different ratios of BaO additions in the solid phase pellet is successfully fabricated by the modified top seeded infiltration growth technique on the basis of previous research. In the present work, the macroscopic feature, microstructure and critical current density (Jc) of the single domain GdBCO bulk superconductor are investigated in detail. From the top view of the surface of the single domain GdBCO bulk superconductor, all of the samples exhibit clearly the fourfold growth sector boundaries on their top surfaces, and spontaneous satellite grains are observed in none of these samples. It can be seen that the different ratio of BaO addition (from 1 wt% to 4 wt%) cannot affect the growth morphology of the single domain GdBCO bulk superconductor. At the same time, for observations of the microstructure, the small test specimens of dimensions about 2 mm2 mm2 mm are cut from the top surface of the singe domain GdBCO bulk superconductors with different BaO doping ratios and at a distance 5 mm away from the seed, then microstructure analysis is performed in the cleavage phane of the test specimen by using scanning electron microscope. For the sample with 1 wt% BaO doping, the derivative phase of GdBCO (Gd123ss) is found in the Gd123 superconducting matrix. To detect the atomic ratio of the Gd123ss, energy dispersive X-ray spectrometry measurement is carried out on the samples. It is shown that the atomic ratio of the Gd123ss phase is Gd:Ba:Cu=1.566:1.459:3, which proves successfully that the Gd123ss is a kind of phase with lack of barium. With the increase of BaO doping, the phenomenon of lack of barium is effectively controlled and, the nano Gd123ss phase is generated as the flux pinning centre which can be used to improve the superconducting properties in the growth process of GdBCO bulk superconductor. This results can be concluded that the proper doping ratio can control the element substitution effectively, and the solid solution phase can be greatly reduced to some extent, and the critical current can be improved to a certain extent when the amount of BaO added ranges from 2 wt% to 4 wt%, which is very helpful in inhibiting the Gd/Ba substitution and fabricating the high-quality single domain GdBCO bulk superconductors.