Two-dimensional (2D) organic-inorganic lead halide perovskites possess strong spin-orbit coupling, and in the presence of broken inversion symmetry, this may lead to helical excitons and Rashba splitting at the band extrema1-6. However, a direct and systematic measurement of the helical excitons and the Rashba parameter in the system is still lacking. Here, we report distinct two bright co-helical and two dark anti-helical excitons in single crystal ((CH3(CH2)3NH3)2(CH3NH3)n-1PbnI3n+1 (n = 1 to 4), where n determines quantum well (QW) thickness of inorganic layer7. By comprehensively analyzing the fine structure of helical excitons, we find that the Rashba splitting originates from surface inversion asymmetry and surface-normal electric fields, which are determined by the QW’s dielectric environment, n and temperature. The Rashba splitting parameter is found among the highest recorded of 2.66 and 2.5 eV∙Å for the conduction and valence band of n=1 Iodide, respectively, and reduces for n>1 Iodide. Our result shows the importance of helical excitons and Rashba splitting and presents a direct method to quantify the Rashba parameter in complex halide perovskites. We hope that our study inspires applications of QW materials in novel spintronic devices.