Bismuth based cubic pyrochlore materials have the attractive dielectric properties, especially the dielectric tunable property. The Bi_1.5MgNb_1.5O₇ (BMN) and Bi_1.5Mg_1-xNi_xNb_1.5O₇ (BMN-xNi) (x = 0.1–0.5) ceramics were prepared by solid state reaction method. XPS spectra of probing evidence for displacive disorder in BMN-xNi by compared with pristine BMN, which confirms the presence of two types of oxygen in the structure. The relative Ni content is the fundamental factor affects the disorder degree in the BMN-xNi structure. It demonstrated that Ni²⁺ ions prior to substitute onto the A-site of BMN-xNi ceramics when x < 0.3, then which would turn to substitute on the B-site with further increasing Ni content. Moreover, The dielectric response of BMN-xNi are investigated within the temperature range of −50–150 °C at 1 MHz. All the dielectric constants show a slightly decrease with the increasing temperature in the testing temperature range, while the dielectric losses increase sharply with temperature. In addition, the dielectric constant of BMN-xNi pyrochlores increases with the increases of Ni content when x < 0.3, then shows a decrease with further increasing Ni content. However, the Ni content shows little influence on dielectric loss of BMN-xNi pyrochlores. As a result, the BMN-0.3Ni ceramics present the enhanced dielectric properties.