Abstract: The commercialized poly(3,4-ethylenedioxythiophene):poly (styrenesulfonate) (PEDOT:PSS) is usually used as hole transport layers (HTLs) in tin-based perovskite solar cells (TPSCs). However, the further development has been restricted due to the acidity that could damage the stability of TPSCs. Although the PEDOT:PSS solution can be diluted by water to decrease acidity and reduce the cost of device fabrication, the electrical conductivity will decrease obviously in diluted PEDOT:PSS solution. Herein, potassium thiocyanate (KSCN) is selected to regulate the properties of PEDOT:PSS HTLs from the diluted PEDOT:PSS aqueous solution by water with a volume ratio of 1:1 to prepare efficient TPSCs. The effect of KSCN addition on the structure and photoelectrical properties of PEDOT:PSS HTLs and TPSCs have been systematically studied. At the optimal KSCN concentration, the TPSCs based on KSCN-doped PEDOT:PSS HTLs (KSCN-PSCs) demonstrate the champion power conversion efficiency (PCE) of 8.39%, while the reference TPSCs only show a champioan PCE of 6.70%. The further analysis demonstrates that the KSCN additive increases the electrical conductivity of HTLs prepared by the diluted PEDOT:PSS solution, improves the microstructure of perovskite film, and inhibits carrier recombination in TPSCs, leading to the reduced hysteresis effect and enhanced PCE in KSCN-PSCs. This work gives a low-cost and practical strategy to develop a high-quality PEDOT:PSS HTLs from diluted PEDOT:PSS aqueous solution for efficient TPSCs.