An algorithm for realizing the universal inverse time-lag characteristic in microprocessor-based protection is put forward. As all parameters in the inverse time-lag characteristic are flexible with none required to be fixed in this method， it is a universal one. Based on the mathematical analysis， appropriate conversion and special arithmetic， this realization is of accelerated convergence in calculation. Not only is the arithmetic of high operational accuracy， but also it takes less calculation time because of its smaller calculation amount. Furthermore it doesn’t need hardware memory chips such as EEPROM to store tabular data. An error analysis is also made， which shows that the method is suitable for application for its excellent accuracy.