In view of the uncertainty of flood and drought disaster systems, a flood and drought disaster risk system, consisting of the disaster hazard, exposure of disaster receptors, damage sensitivity, and disaster resistance capacity, is proposed, and its quantitative characterization is examined. From the methodology point of view, a method system for flood and drought disaster risk assessment, consisting of five methods: disaster hazard analysis, vulnerability analysis, loss risk analysis and assessment, risk evaluation, and risk decision analysis, was established. The results obtained from these methods in empirical research done by the authors are summarized and analyzed. The results indicate that the method system has universality and effectiveness and can provide a reference for risk assessment of different natural disasters, that the physical mechanism of the method system should be further analyzed in the future, and that these risk assessment methods for flood and drought disasters should be enriched, improved, and perfected, especially the loss risk curve assessment method based on the risk process.