In contrast to the conventional MX2 with mirror symmetry, Janus MXY (M = Mo, W; X = S, Se; Y= S, Se; X≠Y) breaks the mirror symmetry along the direction perpendicular to the two-dimensional material plane, reducing the overall symmetry. The electronegativity differences between X and Y produce an intrinsic electric dipole and cause a built-in electric field, resulting in strong spin-orbit coupling (SOC). Using first-principles density functional theory (DFT) calculations, we investigated the spin-orbit coupling induced by changing the doping ratio x on the surface of the monolayer (SxSe1-x)MSe. The band structures of the monolayer (SxSe1-x)MSe with different S/Se ratios were investigated with and without SOC. In addition to Zeeman-type spin splitting and valley polarisation at the K point, Rashba-type spin splitting around the point Γ was observed. Moreover, Rashba parameters αR of monolayer (SxSe1-x)MSe can be tuned by changing the doping ratio x of the surface.