Abstract: The availability analysis which combines universal generating function (UGF) with Markov state transition theory was proposed to nuclear power system with performance degradation and multi-level support capacity. The system logical model and performance state combination model were constructed by UGF. Furthermore, the computational expression for component state probability based on Markov method was constructed considering performance degradation and multi-level support capacity. And then the model was embedded into the logic model and performance state combination model by UGF. This combined model was used in availability analysis of an actual example with different performance demands, and the influence of maintenance condition to system availability was also analyzed. The results show that this method captures the operational and maintenance characteristics. The obtained results can provide guidance to nuclear power system operation, management, maintenance decision-making and support condition analysis.