Abstract: In real-time control system，there are some uncertain factors caused by concurrency and resource sharing，which lead to uncertion varitions of periodic activities running time，so the real-time control system becomes unstable.In order to ensure high performance and predictability，reducing jitter is very important in such system.By introducing deadline reduction factor，this article determines a new relative deadline，thus better explores the available free time and improves the stability of the system.The experimental results prove that the method used in the literature is more effective and available than the other methods.