The protonation state of the asp dyad is significantly important in revealing enzymatic mechanisms and developing drugs. However, it is hard to determine by calculating free energy changes between possible protonation states, because the free energy changes due to protein conformational flexibility are usually much larger than those originating from different locations of protons. Sophisticated and computationally expensive methods such as free energy perturbation, thermodynamic integration (TI), and quantum mechanics/molecular mechanics are therefore usually used for this purpose. In the present study, we have developed a simple thermodynamic approach to effectively eliminating the free energy changes arising from protein conformational flexibility and estimating the free energy changes only originated from the locations of protons, which provides a fast and reliable method for determining the protonation state of asp dyads. The test of this approach on a total of 15 asp dyad systems, including BACE-1 and HIV-1 protease, shows that the predictions from this approach are all consistent with experiments or with the computationally expensive TI calculations. It is clear that our thermodynamic approach could be used to rapidly and reliably determine the protonation state of the asp dyad.