Vertically oriented highly crystalline 2D layered (BA)2(MA)n−1PbnI3n+1 (BA = CH3(CH2)3NH3, MA = CH3NH3, n = 3, 4) perovskite thin‐films are fabricated with the aid of ammonium thiocyanate (NH4SCN) additive through one‐step spin‐coating process. The humidity‐stability of the film is certified by the almost unchanged X‐ray diffraction patterns after exposed to humid atmosphere (Hr = 55 ± 5%) for 40 d. The photovoltaic devices with the structure of indium tin oxide(ITO)/poly(3,4‐ethylenedioxythiophene):poly(styrene‐sulfonate)/(BA)2(MA)n−1PbnI3n+1(n = 3,4)/[6,6]‐phenyl‐C61‐butyric acid methyl ester/Bathocuproine/Ag are fabricated. The devices based on (BA)2(MA)2Pb3I10 perovskite (n = 3) with the precursor composition of BAI:methylammonium iodide:PbI2:NH4SCN = 2:2:3:1 (by molar ratio) show an averaged power conversion efficiency (PCE) of 6.82%. In the case of (BA)2(MA)3Pb4I13 (n = 4), a higher PCE of 8.79% is achieved. Both of the unsealed devices perform unique stability with almost unchanged PCE during the period of storage in purified N2 glove box. This work provides a simple and effective method to enhance the efficiency of the 2D perovskite solar cell.