Abstract: An obvious character of mathematical model for studying reliability of stochastic CIMS is that the parallel structure of working stations makes the number of system-states increase with exponential growth,and the parallel structure contains limited buffers. This makes system state have a tendency of explosion.Therefore,the exact analysis of the system is in contradiction with the tendency of explosion of the system,which is difficult to be solved.How to solve the problem is just the main topic that CIMS stochastic theory investigates. In the paper, we introduce a diffusion approximation method into the mathematical model of reliability of CIMS with parallel multi-server,and describe complicated process with state explosive tendency of the system by employing a simple continuous process. We give some clear expressions of up estimation boundaries and down estimation boundaries for availability, downwork frequency and production rate of the repairable CIMS under steady-state. Finally we give a numerical example to show that the estimation of boundaries is realized easily.