A feasible method of combining the concept of fluorescence half-life and the power dependent photobleaching rate for characterizing the practical photostability of fluorescent proteins (FPs) was introduced. Furthermore, by using a fluorescent photostability standard, a relative comparison of the photostabilty of FPs from different research groups was proposed, which would be of great benefit for developing novel FPs with optimized emission wavelength, better brightness, and improved photostability. We used rhodamine B as an example to verify this method and evaluate the practical photostability of a far-red FP, mKate-S158C. Experimental results indicated good potential of this method for further study.