It is hard to set up catastrophe potential function by using conventional methods and choose control variables of catastrophe potential function. So the method of setting up potential function is given to analyze the brittleness of complex system by means of relations among systems. The feasibility of analyzing complex system by using catastrophe theory is also discussed. Based on the data come from authority department, brittleness potential function is set up through the relation among systems. Then the concrete process of derivation and the results of analysis are given. And the conclusions, which are conformed to reality, are got.