Abstract: Algorithms are concurrently born with electronic computers．Sometimes we even believe algorithms’ birth is long before that of modern computers．Various algorithms were designed to solve miscellaneous concrete problems．Nevertheless,whether the time complexity of an algorithm’s lower bound exists and if so how to determine that lower bound do not usually come to be a problem worth researching or being valued．A method of modeling and analyzing the time complexity’s lower bound of some classical algorithms wase proposed based on decision model and information theory and how to calculate it was presented to solve the problem．The method provided is feasible and effective for the classical algorithms listed and is also applicable to those not listed we believe.

Key words: Algorithm,Time complexity,Decision tree,Information theory,Lower bound