Failures are very common during the online real-time monitoring of large quantities of complex liquids in industrial processes, and can result in excessive resource consumption and pollution. In this study, we introduce a monitoring method capable of non-contact original-state online real-time monitoring for strongly coated, high-salinity, and multi-component liquids. The principle of the method is to establish the relationship among the concentration of the target substance in the liquid (C), the color space coordinates of the target substance at different concentrations (L^∗, a^∗, b^∗), and the maximum absorption wavelength (λ_max); subsequently, the optimum wavelength λ_T of the liquid is determined by a high-precision scanning-type monitoring system that is used to detect the instantaneous concentration of the target substance in the flowing liquid. Unlike traditional monitoring methods and existing online monitoring methods, the proposed method does not require any pretreatment of the samples (i.e., filtration, dilution, oxidation/reduction, addition of chromogenic agent, constant volume, etc.), and it is capable of original-state online real-time monitoring. This method is employed at a large electrolytic manganese plant to monitor the Fe³⁺ concentration in the colloidal process of the plant’s aging liquid (where the concentrations of Fe³⁺, Mn²⁺, and (NH₄)₂SO₄ are 0.5–18 mg·L⁻¹, 35–39 g·L⁻¹, and 90–110 g·L⁻¹, respectively). The relative error of this monitoring method compared with an off-line laboratory monitoring is less than 2%.