In practical engineering application, it is necessary to consider that the system performance may be affected by various factors such as temperature, pressure, flow, power and so on to cause different demands of system performance. According to the phenomenon that the system performance is constrained by the performance level of work requirements, the traditional Markov process is improved, and the improved Markov process is proposed. Under the condition of taking into account the performance level constraint, the improved Markov process is used to analyze the reliability of multistate series parallel system, and a simple example analysis is given. The results show that compared with the traditional reliability analysis methods, this method greatly reduces the computational complexity.