Readily releasable pool (RRP) of synaptic vesicles as the concept that defines the capacity of vesicle availability has been widely used in the study of synaptic transmission. Several approaches have been developed to determine the size of RRP by extrapolating the cumulative EPSCs corresponding to a train stimulation based on the assumption that the properties of vesicle in this pool are homogeneous. However, the estimated RRP sizes by these approaches often vary with different stimulation frequencies. The frequency dependence displays the incompletion of estimation and contradictory to the definition of RRP. Here, we proposed an improved approach to estimate RRP size. Given the replenishment rate proportional to the released part of RRP and corrected the unreleased part of RRP, we obtained a complete mathematical description of the RRP release process and derived the analytic solution. Compared with the two commonly used methods, it has been found that this scheme well describes the dynamic process of RRP, and obtains a relatively good evaluation of RRP size and release probability. This approach is not limited by the stimulation frequency and can be well applied to the cells that cannot be stimulated by high frequency.