Surface passivation is a promising technique to improve the power conversion efficiency (PCE) and stability of perovskite solar cells (PSCs). Upper surface passivation has been widely used to passivate surface traps responsible for interfacial recombination loss. The bottom surface (or substrate) passivation has also been studied using similar additives but with lesser improvement. Here we investigated the passivation effect of SnO substrates by two potassium salts with different functional sulfate anions, namely K-FSI and K-OTf. From calculation and experimental data, we found that anions play three roles in substrate passivation: removal of the hydroxyl group on SnO substrates, improving perovskite crystallinity and promoting electron extraction. The OTf anion is more effective than FSI due to its larger adsorption energy on the SnO substrate. K-OTf passivation also improves interfacial contact and energy level alignment, but K-FSI passivation induced an upward band bending from workfunction mismatch. As a result, K-FSI salts passivation only slightly increase the PCE from 21.00% to 21.86%, while the K-OTf further increases PCE to 22.33%. Our study suggests a viable method of bottom surface passivation by selecting a proper combination of anions and cations.