Transparent relaxor ferroelectrics, (K0.5Na0.5)(0.9)Sr0.1Nb0.9Ti0.1O3-x%Bi2O3 (x = 0, 0.5, 1.0, 1.5, and 2, abbreviated as KNSNT-10x), were fabricated using a pressureless sintering method. With the modification of Bi2O3, the crystal phase of KNSNT ceramics transformed from tetragonal to cubic-like symmetry, and dielectric behaviors became more relaxor like. These changes could reduce the light scattering arisen from birefringence and long-range ordering structure, thus enhancing the optical properties. For KNSNT-10 transparent ceramics, fingerprint morphological nanodomains were observed in the transmission electron microscope image, which made the ceramics sensitive to electric field. Therefore, a large effective linear electro-optic coefficient of 26.1 pm/V was obtained for the KNSNT-10 ceramics, and it is higher than the value (19.9 pm/V) for LiNbO3 single crystal.