The effects of a BaO-2B2O3 sintering aid on the microstructural, ferroelectric, dielectric, and piezoelectric properties of CaBi2Nb2O9 were investigated in detail. TEM results confirm the presence of an amorphous phase at the grain boundary and the trace of some Ba elements occupying the Ca sites in the crystal lattice. The result of XPS spectroscopy, Raman spectroscopy, and electrical test system reveals that the addition of a sintering aid can effectively reduce the oxygen vacancies in the ceramics and improve the DC resistivity (in the low-temperature region) and ferroelectric and dielectric properties of the samples. Meanwhile, the addition of BaO-2B2O3 does not decrease the Curie temperature (TC) of the ceramics. The addition of a 1 wt. % BaO-2B2O3 sample possesses the excellent electrical properties with large remnant polarization (Pr = 12.8 μC/cm2), piezoelectric coefficient (d33=16.6 pC/N), and high TC (937 °C). This study shows that the prepared ceramics have great potential in the application of high-temperature piezoelectric materials.