The traditional solid-phase reaction method was used to dope the 0.94(k$_{0.5}$Na$_{0.5}$)NbO$_3$–0.06Sr(Zn$_{1/3}$Nb$_{2/3}$)O$_3$ (0.94KNN–0.06SZN) with rare-earth Er$^{3+}$, showing that the transparent ferroelectric ceramics have both up-conversion luminescence. Also the changes in the phase structure, optoelectronic properties of the ceramics after Er$^{3+}$ doping were investigated. The results show that the doping of Er$^{3+}$ has no significant effect on the phase structure, dielectric constant, coercivity field and residual polarization intensity of the ceramics. With the increase of Er$^{3+}$ content, the saturation polarization intensity shows a trend of decreasing and then increasing, and the dielectric constant first decreases and then stabilizes. The large amount of Er$^{3+}$ also greatly reduced the light transmission of the ceramics. In addition, the doping of Er$^{3+}$ gives the ceramics new properties. Under 980 nm laser excitation, the ceramics exhibit luminescent emission bands at 533, 554 nm (green) and 672 nm (red). The luminous intensity of the ceramic first strengthens with the increase of Er$^{3+}$ content and then weakens, and the strongest luminous intensity is obtained when the Er$^{3+}$ content is 1.00% mol. Transparent ferroelectric ceramics with light-emitting functions will have a broad application prospect in the field of photoelectric crossover.