Abstract: Electrochemical mass spectrometry is an important technique for studying the mechanism of electrochemical reactions. Mass spectrometry has the advantages of high detection sensitivity, high detection throughput and high specificity, which enables simultaneous acquisition of qualitative and quantitative informations for multiple components, making it highly valuable in the monitoring of electrochemical reaction products and the identification of intermediates. In general, electrolyte is necessary for electrochemical reaction. However, these electrolyte salts are detrimental to mass spectrometry analysis as they not only cause condensation in the mass spectrometry inlet, leading to blockage, but also impede the signal of the target analyte or form adduct ions with target analyte, resulting in a complex mass spectrum and increased analytical challenges. It is of great significance to develop a mass spectrometry method to real-time obtain trace organic components in high concentration buffer salt solutions. Therefore, a novel atmospheric pressure chemical ionization mass spectrometry (APCI-MS) method was developed for online analysis of trace organic components in high concentration buffer salt solutions, and a small volume flow electrolytic cell, stable performance, and simple structure were fabricated for on-line electrochemical reaction, which employed Pt, Pt and Ag/AgCl as working electrode, counter electrode and reference electrode, respectively. After combination of the APCI-MS and flow electrolytic cell, an electrochemical mass spectrometry device and method for on-line analysis of the electrochemical process in high concentration buffer salt solutions were constructed. As a proof of concept application, the device and method were used to on-line and real-time study the electrochemical oxidation process of dopamine in phosphate buffer salt solutions. The experimental results showed that the APCI-MS method can efficiently remove the inorganic salts in solutions on-line by on-line thermal deposition, while the organic components can be ionized by APCI for mass spectrometry detection. The results of dopamine electrochemical oxidation analysis showed that dopamine was not oxidized under the oxidation potential at 0 V, and only the protonated dopamine m/z 154 M+H⁺ was obtained. When the oxidation potential was +0.3 V, in addition to the protonated dopamine, the protonated peak m/z 150 M+H⁺ of dopachrome was generated by losing 4e^- from dopamine, and the protonated peak m/z 303 M+H⁺ of the adduct dopachrome and dopamine formed by the intermolecular hydrogen bonding were also obtained. When the oxidation potential increased to +0.6 V, the contents of oxidation products m/z 150 and m/z 303 were further increased. The results demonstrated that this method has great application potential in studying the electrochemical reaction.