Perovskite oxides have attracted much attention owing to their excellent nonlinear optical properties. Moreover, these properties can be strongly affected and regulated by the doping strategy. In this work, a novel solvothermal method was designed to synthesize Er3+ doped single crystal KNbO3 nanosheets. XRD analysis indicated that, with increasing content of Er, Er3+ could firstly substitute into the B site (Nb5+) and then A site (K+) in the ABO3 perovskite structure. The orthorhombic structure of the pure KNbO3 and Er3+ doped KNbO3 was confirmed according to the Raman spectra. The as-obtained products were nanosheets with a thickness of about 70 nm and diameter of 3 μm in terms of the SEM observations. The single crystal structure of as-obtained products was confirmed by HRTEM. Furthermore, the growth mechanism of Er3+ doped KNbO3 nanosheets was rationally elaborated and proposed. The as-obtained Er3+ doped KNbO3 nanosheets exhibited excellent up-conversion (UC) photoluminescence (PL) properties with a strong green emission and a weak red emission under the excitation of a laser with a wavelength of 980 nm. The most outstanding UC PL property was endowed to the KNbO3 nanosheets by the doping of 0.4 mol% Er3+. It was confirmed that two photons contributed to this UC PL behaviors based on the spectra exited by different laser power. As a conclusion, the as-synthesized perovskite KNbO3 nanosheets with excellent UC PL property could be the potential alternative materials for the development of nonlinear optical devices.