Many countries have paid more attention to seismic risk in nuclear power plants recently. Recent U.S. Nuclear Regulatory Commission actions have increased the expectation that major efforts will be required to expand plant Probabilistic Risk Analysis (PRA) models to include seismic events. PRA is a powerful tool to identify the seismic vulnerabilities in a nuclear power plant. This study introduces a method to quantify seismic risk in a nuclear power plant. A special seismic risk quantification tool was developed to perform uncertainty simulation for seismic occurrence frequencies and failure events probabilities in this work. Compared with some other software, some significant improvements were made in simulation scope and data processing method. This study took a nuclear power plant under construction in China as an example. The seismic vulnerabilities were identified and the risk insights were proposed to further reduce the risk. In order to optimize the plant design, a risk-informed seismic PRA was applied in the design for the first time. An optimization strategy is proposed to make the plant more cost effective and competitive in the market. This study provides a very useful guidance for plant design optimization in the future.