This paper demonstrates that the main reaction, mechanism of flue gas desulfurization (FGD) by pulsed corona discharge or electron beam, which generates non-thermal plasmas (NTP), is heterogeneous. The experimental results in this paper suggest that the main reactions in FGD by NTP are activated reactions in liquid phase. According to Huie's mechanism, it is considered that the ionic radical SOB plays a major part in oxidizing sulfite ions to sulfate ions. Kinetic analysis proves the existence of prior equilibrium and hints that improvement of absorption of gas radicals may promote performance. Solving the kinetic equations with a suggested effective area factor phi reveals that the hydroxyl radicals absorbed in the liquid phase may contribute to over 80% of the overall desulfurization rate. Both theoretical and experimental correlations between desulfurization rate, energy yield and the operating parameters have been obtained, showing satisfactory agreement with each other.