The principal role of the Auxiliary Feedwater System(ASG) is to support the removal of stored and decayed heat from the Reactor Coolant System (RCS). After the Three Mile Island (TMI) accident, the ASG modification for the mitigation of the accidents of the nuclear power plants (NPPs) becomes a research issue. In order to improve the overall safety of nuclear power plants (NPP) and the system reliability, the ASG is renovated with the improved safety facility (ESF) for the Fuqing NPP. A fault tree (FT) model of the ASG before/after the improvement is built by the Probabilistic Safety Analysis (PSA) to analyze the system reliability. The β factor model is used to quantitatively analyze the failures of the components and the minimal cutsets of the FT are calculated. The results show that the probability of the ASGP0200 and the ASGP0300 as the top key event is significantly reduced; and the percentage of the turbine driven pump minimal cutset probability in the top key event probability is also significantly reduced. The initial event tree (ET) model related to the ASG before/after the improvement is built to analyze the contribution of the improvement on the CDF under the power operation condition. The percentage of the initial event contribution on the CDF before/after the ASG improvement is also investigated. The results show that because of the ASG design improvements, the CDF of the NPP is decreased, which means that the safety of the NPP is improved.