The key to the reliability of CIMS production lines is to analyze the states of buffer. When the capacity of buffer increases, the complexity of analyzing the production line increases too. This paper introduced a state-simplifying method. First, the states of the buffer and the work stations are analyzed, the K+1 states of buffer, whose capacity is K, are simplified into three states: empty, non-empty, non-full and full. The reliability of CIMS production line was analyzed and investigated under four cases of the work stations. Three theorems of the state-simplifying were proposed and the state transition rate of the three simplified states were obtained.