The defects on the surface of low-temperature-processed electronic transport layers hindered the development of efficient flexible perovskite solar cells. Herein, we develop a universal NdCl3 dosing strategy to circumvent the residual Sn(II)-OH defects from the incomplete wet-chemical reaction. The introduction of NdCl3 does not lead to the doping of Nd3+ ions but rather the formation of a composite film of NdCl3 with SnOx. The dose of NdCl3 effectively reduces surface trap states at low-temperature-processed SnOx films, leading to increased carrier extraction and reduced carrier accumulation/recombination at the ETL/perovskite interface. These improvements result in perovskite solar cells (PvSCs) with significantly enhanced power conversion efficiency (PCE) and eliminated hysteresis. Finally, efficiencies of 18.62% and 21.49% for PvSCs based on MAPbI3 and FA1-xMAxPbI3 perovskites, respectively, were achieved on rigid substrates. The test on a flexible device based on Cs0.05(FA0.83MA0.17)0.95(I0.83Br0.17)3 perovskite realized a PCE of 16.14% and an incredible VOC of 1.158 V. This study indicated the potential of NdCl3 dose as a universal approach to enhance the performance of PvSCs with low-temperature-processed SnOx ETL.