This chapter investigates the stability of high arch dams during the initial impoundment period, combining microseismic monitoring and numerical simulation techniques. Centered on the Dagangshan high arch dam, the study begins with an overview of the research background, significance, and a review of existing methods, including numerical analysis, safety monitoring, and microseismic applications for high arch dams. Key challenges in current research are identified, forming the basis of the study. The deformation mechanism at the dam heel is analyzed using microseismic monitoring to reveal the spatial and temporal characteristics of seismic events and their role in initial impoundment-induced deformation. Crack formation and propagation in dam galleries are also studied through integrated microseismic monitoring and numerical simulation, offering insights into crack initiation and growth. Additionally, the chapter evaluates the overall working performance of the high arch dam during the impoundment period by synthesizing microseismic data and simulation results. This comprehensive analysis provides a deeper understanding of the stability mechanisms and offers theoretical and practical guidance for ensuring safety and optimizing design and monitoring strategies for high arch dams during critical operational phases.