Abstract: The digital I＆C system（DCS） is the nerve center of nuclear power plant, which plays a vital role in the safe and economical operation of the unit. The I/O cards are key components for the DCS to interact with field instruments. When the I/O cards fail, they need to be replaced in time to ensure the safe operation of nuclear power plant. Therefore, a certain number of spare parts are required for the I/O cards. If the spare parts are rather few, the system availability cannot meet the requirements, otherwise the total cost will be too high.Based on the Markov model, an optimization method for the number of the I/O card spare parts under the condition of availability constraint and considering common cause failure is proposed. The probability detector PRSIM is used to analyze the Markov model quantitatively,and the minimum number of spare parts satisfying the availability constraints is obtained.