A novel method for fast simulation of discharge, which uses a plasma fluid model, is proposed with a focus on the small space-charge variation in the calculation of electric field. A simple scalar equation is employed in the proposed method to calculate the electric field variation instead of solving Poisson's equation at every time step when the dielectric relaxation time is relatively large compared to the time step interval. We perform numerical simulations of surface dielectric-barrier-discharge at atmospheric pressure, which is utilized in developing an active flow control device referred to as a plasma actuator, and demonstrate significant reduction of solve count of the Poisson's equation throughout the simulation. The simulation result indicates that the CPU time required for the electric-field calculation becomes one order of magnitude smaller than that of the conventional approach within an error margin of one percent when several-10 kHz sinusoidal voltage is applied for the discharge. Moreover, a cost reduction of almost two orders of magnitude is achieved in the lower-frequency case with the same order of the error in the several-10 kHz case. Our approach enables large-scale discharge simulation and simulation of multi-physics phenomenon including the discharge physics with a practical computational cost.